Proximal antecedents and acute outcomes of simultaneous alcohol and cannabis use: Systematic review of daily‐ and event‐level studies
Department of Psychology, York University, Toronto, Ontario, Canada
Department of Population and Public Health Sciences, University of Southern California, Los Angeles, California, USA
Institute for Addiction Science, University of Southern California, Los Angeles, California, USA
Institute for Mental Health Policy Research, Centre for Addiction and Mental Health, Toronto, Ontario, Canada
Department of Psychiatry, University of Toronto, Toronto, Ontario, Canada
Abstract
Many studies have revealed that individuals who engage in simultaneous use of alcohol and cannabis report elevated substance‐related consequences relative to those who use only alcohol or cannabis; however, evidence from emerging studies examining within‐person differences across simultaneous use and single substance use occasions is less consistent. This systematic review aimed to synthesize findings from existing day‐ and event‐level studies of within‐person differences in the proximal antecedents and acute outcomes associated with simultaneous use versus single substance use episodes. Our search strategy revealed 30 eligible articles. Two categories of antecedents (i.e., internal [e.g., motives] and external [e.g., social context]) and three categories of outcomes (i.e., consumption behavior, general positive and negative consequences, and specific consequences) were identified. The current literature consistently suggests that greater day‐ or event‐level social and enhancement motives, as well as being in a social context, predict greater likelihood of engaging in simultaneous use compared with alcohol‐ or cannabis‐only use. However, there was heterogeneity in findings regarding the role of other person‐level antecedents. Further, while most evidence pointed to heavier alcohol consumption on simultaneous use occasions versus alcohol‐only occasions, findings for elevations in acute negative and positive substance‐related consequences on simultaneous use versus single substance use occasions were mixed. Additionally, four studies found that increased consequences on simultaneous use occasions depended on the level of alcohol consumed. This review identifies several antecedents for simultaneous use events but suggests that simultaneous use occasions are not always associated with more acute harms than single substance use occasions. Given the extent to which the current literature is mixed, this review emphasizes the importance of methodological improvements and future research examining the mechanisms linking simultaneous use with substance‐related consequences to help reconcile findings across within‐person and between‐person studies.
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Keywords: alcohol, co‐use, ecological momentary assessment, marijuana, SAM
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This systematic review of the proximal antecedents and acute outcomes of simultaneous alcohol and cannabis use found that social and enhancement motives and social contexts were consistently associated with increased likelihood of engaging in simultaneous use versus single substance use. Whereas studies consistently observed greater alcohol consumption on simultaneous use relative to single substance use days/events, findings for differences in acute positive and negative substance‐related consequences were mixed, with methodological differences across studies likely contributing to heterogeneity in the results.
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Received 2024 Apr 29; Accepted 2024 Oct 18; Issue date 2025 Jan.
INTRODUCTION
Alcohol and cannabis are two of the most commonly used substances globally (World Health Organization, 2022, n.d.). Prevalence of simultaneous alcohol and cannabis use, typically defined as using both alcohol and cannabis at the same time and/or with overlapping effects (Lee et al., 2022), has increased in recent years (Gonçalves et al., 2023; Hobin et al., 2023). Much of the current literature has focused on young adults due to particularly high rates of simultaneous use in this population (Gonçalves et al., 2023), with young adults who are younger (i.e., 19–22 years) showing greater prevalence of simultaneous use than young adults who are older (i.e., 27–30 years; Terry‐McElrath & Patrick, 2018). As there are risks for harms associated with using alcohol and cannabis independently (Iranpour & Nakhaee, 2019; Marconi et al., 2016; Wong et al., 2019), it is important to understand how using alcohol and cannabis together could lead to elevated consequences compared with using either substance on its own.
There is accumulating evidence from between‐person analyses that individuals who engage in simultaneous use report more negative consequences relative to individuals who only use alcohol or cannabis (for reviews see Lee et al., 2022; Yurasek et al., 2017). Further, individuals who engage in simultaneous use report greater alcohol‐related consequences than those who engage in concurrent use of alcohol and cannabis (Jackson et al., 2020), the latter being defined as using both substances but at different times (Subbaraman & Kerr, 2015). Also, prevalence of simultaneous use in adults is nearly double the prevalence of concurrent use (Subbaraman & Kerr, 2015), lending support to the importance of focusing on simultaneous use. Building on between‐person studies comparing those who engage in simultaneous use to those who do not, there has been a surge in research in the past 5 years examining how proximal antecedents and acute consequences differ within persons across simultaneous use and single substance use occasions, with findings from these studies being less consistent. Thus, the goal of this review was to summarize these recent within‐person studies and to synthesize the extant knowledge on proximal antecedents and acute consequences of simultaneous use compared with single‐substance use.
Between‐person studies of simultaneous use
Many studies have compared individuals who consume both alcohol and cannabis to those who consume only one of these substances (Lee et al., 2022). These between‐person studies tend to show that individuals who engage in simultaneous use consume more alcohol while drinking, consume alcohol more frequently, and endorse more frequent binge drinking episodes than those who use only alcohol (Cummings et al., 2019; Linden‐Carmichael et al., 2019; Subbaraman & Kerr, 2015). Individuals who report simultaneous use also endorse greater negative alcohol‐related consequences (e.g., driving while under the influence, social consequences, and more time spent drinking) than those who consume only alcohol (Briere et al., 2011; Jackson et al., 2020; Linden‐Carmichael et al., 2019; Subbaraman & Kerr, 2015). Fewer studies have examined differences between those who engage in simultaneous use versus those who engage only in cannabis use, and findings are mixed, with some evidence that simultaneous use is linked with more frequent and heavier cannabis use than cannabis‐only use (e.g., Linden‐Carmichael et al., 2019), and other studies finding no such difference (e.g., Looby et al., 2021). Additionally, there is evidence that those who engage in simultaneous use experience more negative consequences than those who engage in concurrent use, although differences in frequency of alcohol use may help to explain these findings (Cummings et al., 2019; Jackson et al., 2020).
Several studies have also investigated individual differences that predict who is most likely to engage in simultaneous use. Some key predictors that have been identified are using alcohol or cannabis more frequently, having substance‐using peers, and greater simultaneous use motives (Briere et al., 2011; Lee et al., 2022; Terry‐McElrath et al., 2013). Certain demographic (e.g., identifying as White, male sex, younger age), personality (e.g., sensation seeking), and behavioral factors (e.g., earlier age of alcohol and cannabis use initiation, using cannabis more often at parties, and greater truancy in school) also are associated with an increased likelihood of engaging in simultaneous use compared with alcohol‐only use or concurrent use (Cummings et al., 2019; Linden‐Carmichael et al., 2019; Looby et al., 2021; Terry‐McElrath et al., 2013; Waddell, Blake, & Chassin, 2021). In summary, the between‐person literature has established that there are individual differences between those who engage in simultaneous use and those who engage in single substance or concurrent (nonsimultaneous) use, in terms of both risk factors for engaging in use and substance‐related outcomes.
Within‐person studies of simultaneous use
Compared with the number of between‐person studies, until recently, there has been less research on the proximal antecedents and acute outcomes that differ within‐person across simultaneous use and alcohol‐only or cannabis‐only occasions. However, there has been a recent surge of daily diary and ecological momentary assessment (EMA) studies published in the last 5 years that have examined within‐person comparisons between simultaneous and single‐substance use occasions in terms of proximal antecedents (e.g., motives) and acute outcomes (e.g., negative consequences). These within‐person analyses are crucial because drawing inferences about within‐person processes from between‐person associations can be problematic (Curran & Bauer, 2011). For example, the finding that individuals who engage in simultaneous use experience more negative substance‐related consequences overall relative to individuals who engage only in alcohol use does not establish whether these consequences are in fact acute outcomes of simultaneous use behavior or whether they are driven by greater substance use in general or some other third variable (e.g., dispositional factors associated with simultaneous use propensity).
It is believed that the risks of simultaneous use occur in part due to additive or interactive psychopharmacological effects of alcohol and cannabis; specifically, combining the two substances may increase intoxication, and cognitive and behavioral impairment, relative to using either substance on its own (Fares et al., 2022; Lukas & Orozco, 2001; Yurasek et al., 2017). Within‐person analyses can help to isolate acute consequences that are temporally linked with simultaneous use (vs. single substance use) at the event‐level (e.g., risky behaviors that occur because of heightened intoxication when cannabis is combined with alcohol) from broader substance‐related problems among those who engage in simultaneous use that may not be attributable to the specific effects of combining cannabis and alcohol (e.g., academic problems that accumulate from a pattern of heavier alcohol and cannabis use in general). As the literature has shown that there are behavioral and personality trait differences (e.g., truancy in school and impulsivity; Terry‐McElrath et al., 2013; Waddell, Blake, & Chassin, 2021) between those who engage in simultaneous use and those who engage in single substance use, it may be that these individual difference factors contribute to increased substance‐related consequences among those who engage in simultaneous use, which can be controlled using within‐person designs. Further, EMA studies that capture momentary data at the time substances are used have helped to clarify whether there are within‐person differences in the event‐level antecedents of substance use (e.g., contexts, motives) between simultaneous use events and single‐substance events that could be important targets of harm reduction interventions.
Whereas prior between‐person studies have consistently shown evidence that individuals who engage in simultaneous use report elevated negative consequences relative to those who only engage in single‐substance use, the evidence surrounding within‐person differences in consequences across simultaneous use and single substance use occasions is less consistent (see Lee et al., 2022 for a previous review). Further, given the rapid rate with which the within‐person literature has been evolving, it is important to synthesize this emerging within‐person research to provide clarity in the themes of the existing findings and to reveal avenues for future research.
The present review
Several previous reviews have sought to characterize predictors and consequences of simultaneous alcohol and cannabis use (e.g., Karoly et al., 2020; Lee et al., 2022; Schlienz & Lee, 2018; Yurasek et al., 2017), but these reviews have summarized the literature on simultaneous use more broadly, focusing primarily on between‐person and laboratory‐based studies. As many of the current EMA and daily diary studies were published after these prior reviews were conducted, there remains a need for a detailed summary that focuses specifically on within‐person comparisons across simultaneous and single substance use days/events. Additionally, although Lee et al.'s (2022) more recent review includes a number of day‐ and event‐level studies, not all of the studies they reviewed compared simultaneous use to the use of alcohol‐ or cannabis‐only, and their review did not specifically focus on characterizing within‐person differences in the proximal antecedents and acute outcomes linked with simultaneous use. This is important to improve our understanding of whether heightened risk for consequences of simultaneous use is temporally linked with engaging in simultaneous use (vs. single substance use) rather than being driven by individual differences between those who engage in simultaneous use and those who do not. Further, as there have been some inconsistencies in daily‐ and event‐level findings, a review that synthesizes within‐person comparisons will aid in examining potential measurement/methodological issues in the literature that may play a role in these discrepant findings. Given the speed with which the daily dairy and EMA literature examining simultaneous use is evolving, we aimed to provide a timely review that focused specifically on studies that used daily‐ or event‐level designs to examine within‐person comparisons between simultaneous use versus single‐substance days/events with respect to proximal antecedents and acute outcomes.
MATERIALS AND METHODS
The present review was preregistered on PROSPERO (registration ID: CRD42023425606) and aligns with the Preferred Reporting Items for Systematic Review and Meta‐Analyses (PRISMA; Page et al., 2020). The search was conducted using the databases PsycINFO, PubMed, EMBASE, and Web of Science through April 2023. In May 2024, the search was updated to capture studies published between April 2023 and May 2024. Relevant studies were identified using the Boolean search terms ("ecological momentary" OR "EMA" OR "daily‐diar*" OR “daily diar*” OR “daily survey*” OR “daily” OR “burst” OR "experience sampling" OR “timeline follow back” OR “timeline followback” OR “TLFB” OR “event‐level” OR “event level” OR “day‐level” OR “day level”) AND ("cannabis" OR "marijuana") AND ("alcohol" OR “drink*”) AND ("simultaneous" OR "co‐us*" OR "cous*" OR "concur*" OR “crossfad*” OR “cross‐fad*” OR “cooccur*” OR “co‐occur*”). Given that multiple definitions of simultaneous use appear in the literature, we included studies with a variety of operationalizations of simultaneous use, including the use of both alcohol and cannabis “with overlapping effects” and the use of both substances at the same time or within a narrow time window (e.g., within the same 2‐h period). We also included studies that examined use of both alcohol and cannabis within the same day but not necessarily with overlapping effects (typically referred to as “same‐day co‐use”; Lee et al., 2022). We included studies of same‐day co‐use to allow for a wider scope of eligible studies, consistent with the approach taken in the most recent review of the simultaneous use literature (Lee et al., 2022). Additionally, prior studies have shown that the majority of same‐day co‐use days among young adults involve use of both alcohol and cannabis in a narrow time window (i.e., within a few hours; Sokolovsky et al., 2020; Wardell et al., 2024), suggesting that same‐day co‐use largely reflects simultaneous use (according to time‐based definitions), at least among young adults. For the purpose of this review, we synthesized findings comparing simultaneous use (either at the day or event‐level) to alcohol‐only or cannabis‐only use. We used the term “alcohol‐only” to refer to days/events where alcohol was used but cannabis was not (and vice versa for “cannabis‐only”). The terms “cannabis‐only” and “alcohol‐only” are not meant to imply that those days/events excluded the use of other substances such as nicotine (as in many studies, use of substances other than alcohol and cannabis is not accounted for at the event/day level).
Two reviewers (TA and ND) independently reviewed abstracts using Covidence (Veritas Health Innovation Inc.) for full‐text screening and inclusion. Any discrepancies were resolved with a third reviewer (KF). Eligibility criteria were as follows: (i) quantitative studies involving human participants, (ii) studies examining real‐world patterns of alcohol and cannabis use at the day‐ or event‐level, either through prospective day/event‐level assessments or retrospective assessments of day‐level use, (iii) studies reporting within‐person comparisons of simultaneous alcohol and cannabis use to alcohol‐ or cannabis‐only use, (iv) studies measuring day‐ or event‐level antecedents and/or outcomes of simultaneous use compared with alcohol‐ or cannabis‐only use, (v) studies published in a peer‐reviewed journal, and (vi) studies published in English. Studies that included retrospective day‐level measures (i.e., Timeline Follow Back [TLFB]) were included as long as they analyzed their data at the day or event level and reported on the within‐person comparisons between simultaneous use days and single substance use days. This was done to provide a comprehensive review of observational studies that have reported within‐person comparisons and allowed us to explore potential discrepancies across different methodological approaches. Articles were excluded if they (i) reported only between‐person analyses, (ii) exclusively involved experimental designs in which dosing of alcohol and/or cannabis was manipulated by the experimenter or consumption of alcohol and cannabis occurred exclusively in a laboratory setting, (iii) included only qualitative methods/analyses, or (iv) reflected a meta‐analysis or other review.
Following screening, data extraction was conducted by three members of the study team (KF, TA, and ND). Relevant information regarding study characteristics and findings was extracted from each article (see Tables 1, 2, 3, 4, 5 for individual study characteristics). Following extraction, studies were organized by antecedents or outcomes of simultaneous use. Eligible studies were further organized by themes that emerged during data extraction and a narrative synthesis was provided for each of the key themes.
| Author | Year | Country | Main predictor | Study design | Sample | Substance use inclusion criteria | Brief summary of findings |
|---|---|---|---|---|---|---|---|
| Armeli et al. | 2024 | USA | Daily stress | Daily Diary | N = 1461 college students (54% women) | Drinking at least 2 times in the past month and no past treatment for alcohol‐related problems | Daily fluctuations in level of academic stress was negatively associated with simultaneous use compared with alcohol‐only use, such that simultaneous use was less likely (compared with alcohol‐only use) on days with higher school‐related stress |
| Arterberry et al. | 2021 | USA | Motives for alcohol use and motives for cannabis use (assessed separately) | Daily Diary | N = 897 college students (55.7% female) | Data from a larger study of individuals reporting illicit or prescription drug misuse. Analysis included those who reported alcohol or cannabis use at least once during the study | There were no significant differences in drinking motives on simultaneous use vs. alcohol‐only days. Enhancement and social cannabis use motives were associated with greater odds of simultaneous use compared with cannabis‐only use at the day‐level |
| Carpenter et al. | 2024 | USA | Physical pain | EMA | N = 48 adults age 21+ (50% Female) | Weekly alcohol and/or cannabis use | Average pain over the day was associated with simultaneous use in the continuation phase of use (i.e., after initial use) compared with alcohol‐only (but not cannabis‐only use). Event‐level pain was associated with decreased likelihood of simultaneous use in the initiation phase of use (i.e., first use of the day), but increased likelihood of simultaneous use in the continuation phase compared with both alcohol‐only and cannabis‐only use |
| Jackson et al. | 2021 | USA | Motives for simultaneous use or cannabis use (assessed based on type of use day reported) | EMA | N = 341 college students (52% female) | Past month simultaneous use, defined as using alcohol and cannabis so their effects overlapped | Social and enhancement motives predicted a higher likelihood of engaging in simultaneous use compared with cannabis‐only use. Expansion motives predicted greater odds of engaging in cannabis‐only use compared with simultaneous use. However, the relationship between social motives and greater quantity of cannabis use was stronger for cannabis‐only events than for simultaneous use events |
| Patrick et al. | 2019 | USA | Motives for alcohol and/or cannabis use (assessed generally, not specifically linked to a substance) | EMA | N = 399 young adults (50.9% female) | Past month simultaneous use and drinking at least three times in the past month | Greater coping, enhancement, and conformity motives were associated with increased odds of simultaneous use on a given day compared with using alcohol only. Additionally, elevated social, enhancement, and conformity motives predicted increased odds of simultaneous use on a given day compared with using cannabis only |
| Phillips et al. | 2022 | USA | Momentary anxiety | EMA | N = 109 college students (58.7% female) | Using cannabis at least twice a week with latest use in the past week, and a positive THC urine screen | Elevated reports of momentary anxiety were not significantly associated with odds of engaging in simultaneous use compared with either alcohol‐only or cannabis‐only use at the event‐level |
| Author | Year | Country | Main predictor | Study design | Sample | Substance use inclusion criteria | Brief summary of findings |
|---|---|---|---|---|---|---|---|
| Armeli et al. | 2024 | USA | Day of the week | Daily diary | N = 1461 college students (54% women) | Drinking at least two times in the past month and no past treatment for alcohol‐related problems | Simultaneous use was more likely on weekends (vs. week days) compared with alcohol‐only use |
| Gunn et al. | 2021 | USA | Location and social context of use | EMA | N = 313 college students (53% women) | Past year alcohol and cannabis use | Students were more likely to report being at home for cannabis‐only events compared with simultaneous use events and for simultaneous use events compared with alcohol‐only events. Reports of being at a party were greater for simultaneous use events compared with alcohol‐only events, but reports of being at a bar or restaurant were greater for alcohol‐only events compared with simultaneous use events. Simultaneous use events were more likely to occur on weekend days compared with cannabis‐only events. There was a greater likelihood of reporting being with more people at home for simultaneous use events compared with cannabis‐only events |
| Jackson et al. | 2021 | USA | Location and social context of use | EMA | N = 341 college students (52% female) | Simultaneous use, defined as using alcohol and cannabis so their effects overlapped, in the past month at baseline | Being with a significant other or friends, being at a friend's house, and using with more people who also used cannabis or were intoxicated were all associated with increased odds of engaging in a simultaneous use occasion compared with a cannabis‐only occasion. However, using alone and at home were related to engaging in cannabis‐only use compared with simultaneous use. The associations of being at a friend's house and being with more people with greater cannabis use was stronger for cannabis‐only occasions than for simultaneous use occasions |
| Linden‐Carmichael et al. | 2021 | USA | Location and social context of use | Daily Diary | N = 148 young adults (57% female) | At least one episode of binge drinking in past 2 weeks and past‐month simultaneous use | For individuals under age 21, consuming substances at home was associated with increased odds of a simultaneous use day relative to an alcohol‐only use day. But, for those age 21 and over, being at a friend's house or the outdoors were associated with increased odds of engaging in a simultaneous use day relative to an alcohol‐only day. Also, for individuals 21+, being at a bar or club decreased odds of engaging in simultaneous use compared with alcohol only use. Participating in pregaming and playing drinking games did not increase the odds of simultaneous use in either age group. For individuals age 21+, being alone was associated with decreased odds of a simultaneous use day compared with alcohol‐only day, but there was no significant link between simultaneous use and being alone in those under age 21. For both age groups, the number of people they were with and how intoxicated the others were did not affect the odds of engaging in a simultaneous use day compared with an alcohol‐only use day |
| Author | Year | Country | Main outcome/consequence | Study design | Sample | Substance use inclusion criteria | Brief summary of findings |
|---|---|---|---|---|---|---|---|
| Boyle et al. | 2024 | USA | Alcohol use quantity | Daily Diary | N = 86 young adults (60% female) | Simultaneous use 2+ times in the past month and weekly alcohol use | Individuals reported more standard drinks consumed on simultaneous use days compared with alcohol‐only days. Further, heavy episodic drinking (4+/5+ drinks for females/males) was more likely on simultaneous use days compared with alcohol‐only days |
| Fairlie et al. | 2023 | USA | Alcohol use quantity and hours spent high | EMA | N = 409 young adults (50.9% female) | Past month simultaneous use and drinking at least 3 times in the past month | Individuals consumed more standard drinks on simultaneous use days compared with alcohol‐only days. Additionally, individuals spent significantly more hours high on simultaneous use compared with cannabis‐only days |
| Gunn et al. | 2022 | USA | Drinking rate | EMA | N = 318 college students (47% female) | Past year alcohol and cannabis use | The number of standard drinks consumed at peak hour of drinking was higher on simultaneous use days compared with alcohol‐only days. However, simultaneous use days were not associated with differences in daily rate of drinking compared with alcohol‐only days |
| Ito et al. | 2021 | USA | Alcohol use quantity | Retrospective assessment of day‐level use (TLFB) | N = 375 college students (51.2% female) | No specified substance use inclusion criteria | Individuals consumed 158% more drinks on simultaneous use days compared with alcohol‐only days. There was also a significant interaction between cannabis use and AUDIT score on number of drinks consumed on drinking days, such that the association between simultaneous use (compared with alcohol‐only use) and greater number of standard drinks consumed was stronger for those with lower AUDIT scores |
| Lee et al. | 2020 | USA | Alcohol use quantity | EMA | N = 409 young adults (50.9% female) | Past month simultaneous use and drinking at least three times in the past month | Individuals consumed more standard drinks on simultaneous use days compared with alcohol‐only days |
| Linden‐Carmichael et al. | 2020 | USA | Alcohol and cannabis use quantity | Daily Diary | N = 154 young adults (57.8% female) | Past month simultaneous use and heavy episodic drinking episode in the past 2 weeks | Level of alcohol use and subjective alcohol intoxication did not significantly differ across simultaneous use days compared with alcohol‐only days. Similarly, number of cannabis hits reported and subjective cannabis intoxication did not vary across cannabis‐only days compared with simultaneous use days |
| Sokolovsky et al. | 2020 | USA | Subjective intoxication | EMA | N = 341 young adults (51.3% female) | Past month simultaneous use, defined as using alcohol and cannabis so their effects overlapped | Subjective intoxication was higher on simultaneous use days compared with alcohol‐only days and cannabis‐only days |
| Waddell et al. | 2021 | USA | Alcohol use quantity | Retrospective assessment of day‐level use (TLFB) | N = 139 veterans (7.6% female) | Lifetime cannabis use | Type of use day significantly interacted with impulsive traits, such that individuals with lower levels of positive urgency and lack of perseverance drank less on simultaneous use days compared with alcohol‐only days |
| Waddell et al. | 2023 | USA | Alcohol use quantity | EMA | N = 85 young adults (51.8% female) | Past month solitary drinking, social drinking, and binge drinking (for the broader study). Only those who reported use of alcohol and cannabis in the same day were included in analyses | Greater positive high‐arousal subjective effects on simultaneous use days, versus alcohol‐only days, significantly mediated the association between simultaneous use and heavier drinking |
| Author | Year | Country | Main outcome/consequence | Study design | Sample | Substance use inclusion criteria | Brief summary of findings |
|---|---|---|---|---|---|---|---|
| Boyle et al. | 2024 | USA | Number of positive and negative substance‐related consequences (assessed in general, not specifically linked to alcohol or cannabis) | Daily Diary | N = 86 young adults (60% female) | Simultaneous use 2+ times in the past month and weekly alcohol use | Simultaneous use days were associated with more negative and positive consequences compared with alcohol‐only days. Alcohol quantity moderated this difference, such that on lighter drinking days, simultaneous use was associated with greater positive and negative consequences compared with alcohol‐only use days, but on heavier drinking days, number of positive and negative consequences did not differ across simultaneous use and alcohol‐only days. Simultaneous use days were also associated with more positive and negative consequences compared with cannabis‐only days |
| Fairlie et al. | 2023 | USA | Number of negative alcohol and cannabis related consequences | EMA | N = 409 young adults (50.9% female) | Past month simultaneous use and drinking at least 3 times in the past month | Individuals reported more negative alcohol‐related consequences on simultaneous use days compared with alcohol‐only days. However, simultaneous use days were not associated with number of negative cannabis consequences compared with cannabis‐only days |
| Lee et al. | 2020 | USA | Number of positive and negative alcohol consequences | EMA | N = 409 young adults (50.9% female) | Past month simultaneous use and drinking at least three times in the past month | Individuals reported more positive and negative alcohol‐related consequences on simultaneous use days compared with alcohol‐only days, but the association for negative consequences was no longer significant when controlling for number of drinks consumed on a given day |
| Linden‐Carmichael et al. | 2020 | USA | Number of positive and negative substance‐related consequences (assessed in general, not specifically linked to alcohol or cannabis) | Daily Diary | N = 154 young adults (57.8% female) | Past month simultaneous use and a heavy drinking episode in the past 2 weeks | Individuals reported more negative alcohol‐related consequences on simultaneous use days compared with alcohol‐only days. However, number of positive consequences did not differ across simultaneous use and alcohol‐only days. Individuals also reported more positive and negative cannabis consequences on simultaneous use days compared with cannabis‐only days |
| Mallett et al. | 2019 | USA | Number of negative substance‐related consequences (assessed in general, not specifically linked to alcohol or cannabis) | Daily Diary | N = 461 college students (51.4% female) | Past year alcohol and other drug use | Individuals reported more negative substance use consequences on heavy drinking (4/5+ drinks for females/males) simultaneous use days compared with all other use days (i.e., light‐alcohol‐only, cannabis‐only, light drinking simultaneous use days) except heavy drinking alcohol‐only days. Similarly, there were less reported negative consequences on light drinking alcohol‐only compared only to heavy drinking simultaneous use days and heavy drinking alcohol‐only days |
| Sokolovsky et al. | 2020 | USA | Number of negative substance‐related consequences (assessed in general, not specifically linked to alcohol or cannabis) | EMA | N = 341 young adults (51.3% female) | Past month simultaneous use, defined as using alcohol and cannabis so their effects overlapped | There was no difference in the number of negative consequences on simultaneous use days compared with alcohol‐only days. Individuals reported greater negative consequences on simultaneous use days compared with cannabis‐only days |
| Waddell et al. | 2023 | USA | Number of negative alcohol consequences | EMA | N = 85 young adults (51.8% female) | Past month solitary drinking, social drinking, and binge drinking (for the broader study). Only those who reported use of alcohol and cannabis in the same day were included in analyses | There was an indirect association (at the daily level) between simultaneous use (compared with alcohol‐only use) and greater negative consequences through greater positive high‐arousal subjective effects and greater negative low‐arousal subjective effects |
| Waddell et al. | 2023 | USA | Number of negative alcohol‐related consequences | Retrospective assessment of day‐level use (TLFB) | N = 489 college students (68.7% female) | Past month simultaneous alcohol and cannabis use | Higher alcohol consumption simultaneous use days were associated with more negative alcohol consequences compared with moderate alcohol consumption simultaneous use days, higher consumption alcohol‐only days, and moderate alcohol consumption alcohol‐only days. However, moderate alcohol consumption simultaneous use days were associated with fewer negative consequences than higher consumption alcohol only days, but more negative consequences than moderate consumption alcohol‐only days |
| Wardell et al. | 2024 | Canada | Number of negative substance‐related consequences (13 items were not substance‐specific; three items were) | EMA | N = 150 young adults (64% female) | At least weekly alcohol and cannabis use, and simultaneous alcohol and cannabis use at least two times over the past month | Individuals reported more negative consequences on simultaneous use days compared with both alcohol‐only and cannabis‐only days. Additionally, the relationship between number of drinks consumed and negative consequences was weaker on simultaneous use days during which a relatively larger (vs. smaller) amount of cannabis flower was used |
| Author | Year | Country | Main outcome/consequence | Study design | Sample | Substance use inclusion criteria | Brief summary of findings |
|---|---|---|---|---|---|---|---|
| Brown et al. | 2018 | USA | Interpersonal conflict | Daily Diary | N = 427 freshman males | Drinking 5+ drinks in one sitting two times per month, or weekly alcohol consumption | Simultaneous alcohol and cannabis use days were not associated with increased reports of interpersonal conflict above and beyond use of alcohol‐only or cannabis‐only |
| Drohan et al. | 2023 | USA | Seven individual negative consequences (assessed as consequences of alcohol, cannabis or combined use, depending on substances used) | EMA | N = 341 college students (51.3% female) | Past year alcohol and cannabis use | Students were less likely to report hangovers, but more likely to report driving under the influence, on simultaneous vs. alcohol‐only days. Students were more likely to endorse all negative consequences except driving under the influence and unwanted sex on simultaneous use days compared with cannabis‐only days |
| Fairlie et al. | 2023 | USA | Fifteen individual negative alcohol consequences and ten individual negative cannabis consequences | EMA | N = 409 young adults (50.9% female) | Past month simultaneous use and drinking at least three times in the past month | Simultaneous use days were associated with a greater likelihood of doing something embarrassing, feeling clumsy, having difficulty concentrating, and feeling dizzy compared with alcohol‐only days. Further, simultaneous use days were associated with greater likelihood of feeling clumsy and dizzy compared with cannabis‐only days |
| Graupensperger et al. | 2021 | USA | Sleep outcomes | EMA | N = 409 young adults (50.9% female) | Past month simultaneous use and drinking at least three times in the past month | Individuals reported better perceived sleep across all measured indices on simultaneous use days compared with alcohol‐only days. However, individuals reported poorer sleep quality and worse impact of sleep on daily functioning on simultaneous use days compared with cannabis only days |
| Lewis et al. | 2024 | USA | Hooking‐up | EMA | N = 1009 adolescents and young adults | For those age 18+: drinking at least once a month over the past 6 months. For those ages 15–17: no substance use inclusion criteria applied | Simultaneous use days were not associated with likelihood of hooking‐up (defined as any physically intimate behaviors with someone not currently dating or in a romantic relationship with) compared with alcohol‐only or cannabis‐only use days |
| Linden‐Carmichael et al. | 2021 | USA | Negative affect | Daily Diary | N = 154 young adults (57.8% female) | Past month simultaneous use and heavy drinking episode in the past two weeks | Next‐day negative affect was higher on simultaneous use days compared with alcohol‐only and cannabis‐only days, particularly for women with greater reported baseline anxiety levels |
| Patrick et al. | 2021 | USA | Impaired driving | EMA | N = 408 young adults (51% female) | Past month simultaneous use and drinking at least three times in the past month | Individuals were more likely to report being in a car with an impaired driver on simultaneous use days compared with alcohol‐only and cannabis‐only days. There was also increased odds of driving under the influence on simultaneous use days compared with cannabis‐only days |
| Phillips et al. | 2022 | USA | Anxiety | EMA | N = 109 college students (58.7% female) | Using cannabis at least twice a week with latest use in the past week, and a positive THC urine screen | State‐level anxiety reports were lower following simultaneous use events compared with cannabis‐only events, with associations stronger for those with higher baseline levels of anxiety. There was no difference in level of anxiety between alcohol‐only and simultaneous use events |
| Read et al. | 2021 | USA | Sexual Assault Risk | Daily Diary | N = 174 young adult females | No specified substance use inclusion criteria | The risk for reported sexual assault was higher for simultaneous use days above and beyond the use of alcohol‐ or cannabis‐only |
| Sellers et al. | 2021 | USA | Non‐Suicidal Self Injury (NSSI) | Retrospective assessment of day‐level use (TLFB) | N = 71 adolescents (75% female) | Lifetime alcohol use | Alcohol and cannabis use days were not associated with same‐day NSSI (compared with no‐use days). However, alcohol and cannabis use days interacted so that use of both substances on a given day increased the odds of same‐day NSSI over and above the effect of alcohol‐ or cannabis‐only |
| Sznitman et al. | 2023 | Netherlands | Sleep Health | EMA | N = 80 college students (66% female) | Weekly cannabis use (specifically smoking cannabis joints) | Individuals reported taking more time falling asleep, but fewer early awakenings, on simultaneous use days compared with cannabis‐only days. Individuals reported more awakenings on alcohol‐only days compared with simultaneous use days. Finally, there was no difference in total sleep time for simultaneous use days compared with alcohol‐ and cannabis‐only days |
| Sznitman et al. | 2024 | USA | Sleep Quality | Daily Diary | N = 347 adults aged 21–70 (63.7% female) | Past cannabis use and intent to use cannabis for anxiety relief | Individuals reported better sleep quality on simultaneous use days compared with alcohol‐only days. In contrast, individuals reported worse sleep quality on simultaneous use days compared with cannabis‐only days |
| Waddell et al. | 2023 | USA | Eight domains of negative alcohol‐related consequences | Retrospective assessment of day‐level use (TLFB) | N = 489 college students (68.7% female) | Past month simultaneous alcohol and cannabis use | Higher alcohol consumption simultaneous use days were only associated with greater likelihood of physiological dependence symptoms compared with higher consumption alcohol‐only days. Higher alcohol consumption simultaneous use days were associated with a greater likelihood of all consequences domains (self‐care consequences) compared with moderate alcohol consumption simultaneous use days, with a greater likelihood of all consequence domains compared with moderate consumption alcohol‐only days. Higher consumption alcohol‐only days were associated with a greater likelihood of all consequence domains (except academic‐occupational, lack of self‐care, and diminished self perception consequences) compared with moderate consumption alcohol‐only days. Moderate alcohol consumption simultaneous use days were associated with greater likelihood of blackouts, poor self‐care, impaired control, and social‐interpersonal consequences compared with moderate consumptionalcohol‐only days |
RESULTS
Characteristics of the literature
Our initial search revealed 21 eligible articles. After conducting an updated search, we included an additional nine eligible studies for a total of 30 articles (see Figure 1 for PRISMA diagram). These studies were divided into those examining proximal antecedents (k = 8) or acute outcomes/consequences (k = 23), with one studythat looked at both included in both sections. Among the studies that analyzed antecedents of simultaneous use, two key themes of antecedents emerged: internal (e.g., motives; k = 6) and external (e.g., social context; k = 4) antecedents. Additionally, the studies reporting acute outcomes were further divided into those examining consumption behaviors (k = 9), general substance‐related consequences (k = 9), and specific consequences (k = 14). Studies reporting multiple antecedents and/or outcomes were counted in all relevant categories.
The vast majority of studies took place in the United States (93.33%), with one study taking place in the Netherlands (3.33%) and one in Canada (3.33%). Twenty‐six studies (86.67%) focused on young adults or college students, two studies focused on adults more broadly (6.67%), two included adolescents (6.67%), and one study focused on veterans (3.33%). One study included both young adults and adolescents and was counted in both totals. Fourteen studies (46.67%) recruited individuals who reported simultaneous use, four studies (13.33%) indicated that using both alcohol and cannabis (but not simultaneous use specifically) was an eligibility requirement, 10 studies (33.33%) required use of either alcohol or cannabis (but not both), and two studies (6.67%) did not specify substance use inclusion criteria. Most studies utilized an intensive longitudinal design (i.e., daily‐diary or EMA; k = 26; 86.67%), but four studies (13.33%) utilized TLFB measures where retrospective day‐level reports were collected.
Proximal antecedents
Internal antecedents
Six studies compared internal antecedents of simultaneous use versus single substance use (Table 1). Three of these examined substance use motives, which are typically defined as specific reasons for substance use (e.g., social, enhancement, coping, conformity and expansion motives; Cooper, 1994; Simons et al., 1998). All studies took a unique approach to assessing motives: Arterberry et al. (2021) administered motive items for alcohol and cannabis separately each day, Patrick et al. (2019) used more general questions assessing motives for alcohol and/or cannabis use regardless of which substance was used on a given day, and Jackson et al. (2021) examined event‐level motives (reported following substance use) that were substance‐specific. All three studies found that enhancement and social motives were significantly associated with higher odds of engaging in simultaneous use compared with cannabis‐only use at the day‐ or event‐level (Arterberry et al., 2021; Jackson et al., 2021; Patrick et al., 2019). Patrick et al. (2019) found that conformity motives also predicted simultaneous use (vs. cannabis‐only use) at the day level, whereas Jackson et al. (2021) found that greater expansion motives were associated with higher odds of a cannabis‐only use event versus a simultaneous use event.
Two of these studies also examined comparisons between simultaneous use and alcohol‐only days, with one finding no significant associations between drinking motives and likelihood of engaging in simultaneous use versus alcohol‐only use (Arterberry et al., 2021). Conversely, Patrick et al. (2019) found that greater coping, enhancement, and conformity motives were associated with a higher likelihood of engaging in simultaneous use versus alcohol‐only use at the day‐level. Together, these studies reveal that substance use motives (especially social and enhancement motives) tend to differ within‐person across simultaneous use and single substance use (particularly cannabis‐only) events, and support motives as a proximal antecedent of simultaneous use.
Three studies examined other internal antecedents of simultaneous use. One study found that simultaneous use was less likely than alcohol‐only use to occur on days with higher academic stress, whereas day‐level elevations in interpersonal stress were not associated with likelihood of engaging in a simultaneous versus alcohol‐only use day (Armeli et al., 2024). Another study found that pain assessed at the event level was associated with a decreased likelihood of engaging in a simultaneous use event during the first substance use event on a given day, but with an increased likelihood in engaging in a simultaneous use event when examining subsequent use events in a given day (compared with both alcohol‐ and cannabis‐only use events; Carpenter et al., 2024). Finally, one study found that momentary anxiety was not associated with differences in the likelihood of engaging in simultaneous use compared with alcohol‐ or cannabis‐only use on a given day (Phillips et al., 2022).
External antecedents
Four studies examined location/setting of simultaneous alcohol and cannabis use compared with single‐substance use (Table 2). Two studies found that being at a friend's house was associated with engaging in simultaneous use compared with cannabis‐only use (Gunn et al., 2021; Jackson et al., 2021). While Jackson et al. (2021) only looked at simultaneous vs. cannabis‐only use events, Gunn et al. (2021) also found that being at a friend's house was associated with a greater likelihood of simultaneous use compared with alcohol‐only use. In contrast, being at a bar predicted higher odds of alcohol‐only use compared with simultaneous use (Gunn et al., 2021). Being at a party was associated with a greater likelihood of simultaneous use versus cannabis‐only use at the day level in one study (Gunn et al., 2021), whereas being at home was associated with greater odds of engaging in cannabis‐only use compared with simultaneous use in two studies (Gunn et al., 2021; Jackson et al., 2021). Finally, one study found that the relationship between location and simultaneous use depended on age (Linden‐Carmichael, Allen, & Lanza, 2021). For those under age 21, being at home was associated with greater odds of simultaneous use compared with alcohol‐only use. For individuals age 21 and older, using at a friend's house and using outdoors predicted greater odds—and using in a bar or club predicted decreased odds—of simultaneous use compared with alcohol‐only use at the day level (Linden‐Carmichael, Allen, & Lanza, 2021). Two studies also found that weekend versus weekday context matters, with a greater likelihood of engaging in simultaneous use compared with alcohol‐only or cannabis‐only use observed on weekends relative to weekdays (Armeli et al., 2024; Gunn et al., 2021).
Three of the above studies also examined the social context of simultaneous use compared with single substance use (Gunn et al., 2021; Jackson et al., 2021; Linden‐Carmichael, Allen, & Lanza, 2021). Two of these studies found that being with more people and being in social contexts where more people were using cannabis were each associated with a greater likelihood of simultaneous use compared with cannabis‐only use at the day‐ and event‐levels, while being alone predicted greater odds of cannabis‐only use compared with simultaneous use (Gunn et al., 2021; Jackson et al., 2021). Compared with alcohol‐only use, simultaneous use was less likely to occur in solitary versus social contexts for individuals age 21 and older (Linden‐Carmichael, Allen, & Lanza, 2021). Other contextual factors such as using while pregaming or engaging in drinking games were not significantly associated with odds of simultaneous use compared with alcohol‐only use (Linden‐Carmichael, Allen, & Lanza, 2021). In sum, the above studies suggest that contextual factors can predict simultaneous alcohol and cannabis use compared with the use of alcohol‐ or cannabis‐only.
Acute outcomes
Consumption behaviors
Nine studies examined within‐person differences in alcohol or cannabis consumption on simultaneous use days/events compared with single substance days/events (Table 3). Four studies found that simultaneous use days were associated with consuming more standard drinks compared with alcohol‐only days (Boyle et al., 2024; Fairlie et al., 2023; Ito et al., 2021; Lee et al., 2020). Similarly, heavy episodic drinking (defined as 4+/5+ drinks for females and males, respectively) was more likely on simultaneous use days compared with alcohol‐only days (Boyle et al., 2024). One study found that simultaneous use days were associated with consuming more drinks within a single hour compared with alcohol‐only days, but simultaneous use days and alcohol‐only days did not significantly differ in the daily rate of drinking (the number of drinks divided by the number of hours spent drinking per day; Gunn et al., 2022). One study also found that simultaneous use days were associated with more hours spent high compared with cannabis‐only days (Fairlie et al., 2023). However, another study found no within‐person differences between simultaneous use days and alcohol‐ or cannabis‐only days on either number of standard drinks consumed or number of cannabis hits consumed (Linden‐Carmichael et al., 2020). Notably, this was one of the few studies examining consumption outcomes to control for use of cigarettes/vaping and other substances. Further, while all studies controlled for whether use occurred on a weekend versus a weekday, there was heterogeneity in other between‐person (e.g., sex and age) and within‐person (e.g., day in study) covariates included across these studies.
Although not an index of quantities of alcohol and cannabis consumed per se, two studies examined the related outcome of subjective intoxication (accounting for quantity of alcohol and cannabis use), with one study finding greater subjective intoxication on simultaneous use days compared with both alcohol‐only and cannabis‐only days (Sokolovsky et al., 2020) and one study finding no difference in level of subjective intoxication on simultaneous use days compared with alcohol‐only and cannabis‐only days (Linden‐Carmichael et al., 2020).
Three studies examined variables that may influence or explain the association between simultaneous use and heavier alcohol use. One study examined the moderating role of impulsive traits, finding that individuals high in positive urgency and lack of perseverance drank similar quantities of alcohol on simultaneous use days compared with alcohol‐only days, whereas individuals low on these impulsive traits drank fewer drinks on simultaneous use compared with alcohol‐only days (Waddell, Gunn, et al., 2021). Another study found that within‐person differences in the number of standard drinks consumed across simultaneous use days and alcohol‐only days were larger for individuals with lower levels of alcohol problems (Ito et al., 2021). Lastly, one study found that greater positive high‐arousal subjective effects of alcohol mediated the association between simultaneous use days (compared with alcohol‐only use days) and heavier drinking (Waddell, Corbin, et al., 2023).
General substance use consequences
Nine studies examined differences in general substance‐related consequences (i.e., multi‐item measures of total positive and/or negative substance use consequences) across simultaneous use and single substance occasions, with mixed results (Table 4). Four studies compared the number of negative substance‐related consequences (measured as general consequences not specific to alcohol or cannabis) reported on simultaneous use versus alcohol‐only days, with three finding more negative consequences on simultaneous use days (Fairlie et al., 2023; Linden‐Carmichael et al., 2020; Wardell et al., 2024) and one finding no difference (Sokolovsky et al., 2020). Similarly, four studies compared simultaneous use days with cannabis‐only days, with three finding that simultaneous use days were associated with greater negative consequences compared with cannabis‐only days (Linden‐Carmichael et al., 2020; Sokolovsky et al., 2020; Wardell et al., 2024) and one finding no difference (Fairlie et al., 2023).
Four studies revealed mixed findings for within‐person differences in substance‐related consequences associated with simultaneous versus single substance use based on level of alcohol consumption. One study found that simultaneous use days were associated with more negative alcohol‐related consequences compared with alcohol‐only days but that the association was no longer statistically significant when controlling for the number of drinks consumed (Lee et al., 2020). Further, two studies found that the number of drinks consumed moderated the strength of the association between simultaneous use (vs. alcohol‐only use) and number of negative consequences, such that differences between simultaneous use and alcohol‐only days were weaker on heavy drinking days (Boyle et al., 2024; Mallett et al., 2019). Waddell, McDonald, et al. (2023) categorized alcohol‐only days and simultaneous use days into heavier and moderate drinking days, and found that number of consequences reported at the day‐level varied across these categories (see Table 4 for specific findings).
Four studies also examined positive substance‐related consequences (e.g., felt‐relaxed, in a better mood). One study found that simultaneous use days were associated with more positive alcohol‐related consequences compared with alcohol‐only days, even when controlling for number of drinks consumed (Lee et al., 2020). However, another study reported that their finding for greater positive consequences on simultaneous use days compared with alcohol‐only use days was qualified by a significant interaction with alcohol quantity, such that there was no difference in positive consequences between simultaneous and alcohol‐only use on heavier drinking days (Boyle et al., 2024). One study found no significant difference in positive consequences between simultaneous use days compared with alcohol‐only days (Linden‐Carmichael et al., 2020). Finally, two studies found that simultaneous use days were associated with greater positive consequences compared with cannabis‐only days (Boyle et al., 2024; Sokolovsky et al., 2020).
Specific consequences linked with simultaneous use
Fourteen studies examined specific consequences associated with simultaneous use at the within‐person level (Table 5). Of these, two focused on affective consequences. Linden‐Carmichael, Hochgraf, et al. (2021) found that simultaneous use days were associated with greater next‐day negative affect relative to both alcohol‐only and cannabis‐only use days, whereas Phillips et al. (2022) found that simultaneous use episodes were associated with lower subsequent, same‐day state anxiety relative to cannabis‐only use episodes.
Three studies examined sleep‐related outcomes (Graupensperger et al., 2021; Sznitman et al., 2023, 2024), with all three finding that simultaneous use days were associated with better sleep outcomes compared with alcohol‐only days, but when compared with cannabis‐only days, simultaneous use days were associated with poorer sleep (Graupensperger et al., 2021; Sznitman et al., 2023, 2024). However, these differences were not observed across all sleep indices (Table 5).
Three studies examined within‐person interpersonal consequences. Brown et al. (2018) found that although cannabis and alcohol use each independently predicted subsequent interpersonal conflict, the interaction between alcohol and cannabis use (i.e., simultaneous use) did not predict increases in interpersonal conflict over and above the use of each substance independently. Another study found that simultaneous use days were not associated with a greater likelihood of hooking‐up (defined as physically intimate behaviors without expectation of a romantic relationship) compared with alcohol‐ and cannabis‐only days (Lewis et al., 2024). However, Read et al. (2021) found that simultaneous use was associated with greater risk for sexual assault compared with use of either alcohol‐ or cannabis‐only, and this link was mediated by increased intoxication, and in turn, increased likelihood of proximity to likely offenders.
Five studies examined specific negative substance‐related consequences of simultaneous alcohol and cannabis use. When comparing simultaneous use to alcohol‐only use, simultaneous use was associated with an increased likelihood of impaired driving (Drohan et al., 2023; Patrick et al., 2021), feeling clumsy and dizzy, and doing something embarrassing (Fairlie et al., 2023). In contrast, individuals were less likely to report experiencing a hangover on simultaneous use days compared with alcohol‐only days (Drohan et al., 2023). A retrospective study found that elevated risk of specific alcohol consequences (e.g., academic/occupational, blackouts, and physical dependence) on simultaneous versus alcohol‐only days varied based on level of alcohol consumption (Waddell, McDonald, et al., 2023; see Table 5 for specific findings). When comparing simultaneous use occasions to cannabis‐only use, simultaneous use was associated with increased risk of impaired driving (Patrick et al., 2021), and reports of feeling dizzy or clumsy (Fairlie et al., 2023). Another study found that students were more likely to report all examined consequences, except driving under the influence and unwanted sex, on simultaneous use days compared with cannabis‐only days when controlling for number of cannabis uses in a given day (Drohan et al., 2023).
Finally, Sellers et al. (2021) examined day‐level associations between use of alcohol and cannabis and non‐suicidal self‐injury (NSSI) in the 90 days preceding hospitalization (assessed retrospectively) among adolescents, finding that cannabis and alcohol use interacted such that simultaneous use on a given day was associated with increased odds of same day NSSI over and above the independent effects of alcohol and cannabis use.
DISCUSSION
This review summarized the current literature on the proximal antecedents and acute consequences associated with simultaneous alcohol and cannabis use compared with alcohol‐only or cannabis‐only use. The review is the first to our knowledge to focus specifically on day‐ and event‐level observational studies of within‐person comparisons between simultaneous and single‐substance use occasions. Notably, this review includes 16 recent articles that were not captured in the most recent review of the simultaneous use literature (i.e., Lee et al., 2022), highlighting the rapid rate with which studies of within‐person comparisons have been emerging. The extant literature suggests that there are specific motivational and contextual antecedents that distinguish simultaneous use events from single substance use events. The findings for within‐person differences in acute outcomes across simultaneous use and single‐substance use events were less consistent, although a few key themes emerged. This review also reveals the importance of utilizing consistent measurement and methodological approaches in future studies of proximal antecedents and acute consequences associated with simultaneous use.
Proximal antecedents of simultaneous use
This review found consistent evidence that motives and contexts involving social aspects are associated with greater likelihood of engaging in simultaneous use relative to single substance use (Armeli et al., 2024; Arterberry et al., 2021; Gunn et al., 2021; Jackson et al., 2021; Patrick et al., 2019). In contrast, situations more conducive to solitary use, such as being alone, may be more predictive of cannabis‐only use compared with simultaneous use (Jackson et al., 2021). There was also evidence that some between‐person factors, such as age (Linden‐Carmichael, Allen, & Lanza, 2021), can influence the relationship between context and simultaneous use, suggesting that future studies should continue to explore cross‐level interactions between person‐level factors and event‐level antecedents of simultaneous use.
These findings are consistent with the motivational model of substance use, which posits that motives and surrounding contextual influences drive alcohol and cannabis use behavior (Cooper, 1994; Cox & Klinger, 1988; Simons et al., 1998), suggesting that these same factors are also important for understanding simultaneous use. It is possible that motives for using one substance in a social context may increase the likelihood of exposure to other substances in social settings, thus increasing the likelihood of engaging in simultaneous use; this may explain the finding that greater social and enhancement motives, as well as being in a social setting, are linked specifically with simultaneous use relative to single‐substance use.
In contrast, studies examining other internal antecedents of simultaneous use revealed mixed findings, with academic stress (Armeli et al., 2024) and pain (Carpenter et al., 2024), but not anxiety (Phillips et al., 2022), predicting likelihood of simultaneous use versus single substance use. As each of these antecedents was only examined in a single study, conclusions regarding nonmotive internal antecedents of simultaneous use are premature and require replication efforts. Furthermore, while all studies of proximal antecedents used an EMA or daily diary design, there was variation in populations studied (i.e., young adults vs. adults more generally) and eligibility criteria (i.e., alcohol use at least twice in the past month in Armeli et al. and cannabis use at least twice per week in Phillips et al.), making direct comparisons across studies challenging. However, when looking at proximal antecedents as a whole, results for some antecedents (e.g., motives and social context) still converged despite differences across studies in the measurement of motives and the sample inclusion criteria.
Acute outcomes of simultaneous use
Consumption behaviors
Many studies have examined within‐person differences in alcohol consumption across simultaneous use and single substance use occasions. The evidence points toward a tendency to engage in heavier drinking on simultaneous use days compared with alcohol‐only days (Boyle et al., 2024; Fairlie et al., 2023; Gunn et al., 2022; Ito et al., 2021; Lee et al., 2020). Some possible explanations for this finding include the potential for the combined effects of cannabis and alcohol to increase alcohol demand (Morris et al., 2018), or simultaneous use occurring in settings more likely to lead to heavy drinking such as on weekends or at parties (Armeli et al., 2024; Gunn et al., 2021; Jackson et al., 2021). While all nine studies examining consumption outcomes included weekend versus weekday use as a covariate, the confounding role that social context may play in the relationship between simultaneous use and heavier drinking has received minimal attention. Further, as one study found evidence for greater positive high‐arousal subjective alcohol effects as a mediator of the relationship between simultaneous use and heavier alcohol use (Waddell, Corbin, et al., 2023), there is a need to examine additional mechanisms that may play a role in this relationship. Moreover, few studies have looked at whether cannabis consumption differs across simultaneous use versus cannabis‐only use days, and those that have examined cannabis consumption have used inconsistent approaches to quantifying cannabis quantities (e.g., hours spent high in Fairlie et al., 2023; number of hits in Linden‐Carmichael et al., 2020), which might contribute to mixed findings. Thus, there is a need for studies that more rigorously examine within‐person differences in cannabis consumption using standardized indices (e.g., milligrams of THC).
While the general findings for alcohol consumption behaviors were consistent, it is still important to note some methodological differences between studies that may have contributed to a few mixed findings. For example, many studies in the consumption behaviors section did not account for use of substances other than alcohol and cannabis in their analyses, whereas three studies included both nicotine and other substance use as covariates (i.e., Linden‐Carmichael et al., 2020; Sokolovsky et al., 2020; Waddell, Gunn, et al., 2021), one of which was the only study that reported null findings for differences in consumption behaviors across simultaneous and single substance use days (Linden‐Carmichael et al., 2020). As use of other substances is common among young adults (e.g., nicotine vaping; Schulenberg et al., 2021) and may impact levels of alcohol and cannabis consumption (Zuckermann et al., 2020), it may be important to control for other substance use in future research.
General consequences
While there is consistent evidence from between‐person studies that individuals who use both alcohol and cannabis experience elevated substance‐related consequences compared with those who use only one of these substances (Briere et al., 2011; Green et al., 2016; Linden‐Carmichael et al., 2019; Subbaraman & Kerr, 2015), the evidence was mixed regarding whether using both alcohol and cannabis during the same day/event leads to more negative consequences compared with using only alcohol, and differences may be dependent on the level of alcohol consumed (Boyle et al., 2024; Lee et al., 2020; Mallett et al., 2019; Waddell, McDonald, et al., 2023). Additionally, fewer studies examined positive alcohol consequences, and findings for positive consequences also were mixed (Boyle et al., 2024; Lee et al., 2020; Linden‐Carmichael et al., 2020). Similarly, while several studies did observe greater negative consequences on simultaneous use days compared with cannabis‐only days (Linden‐Carmichael et al., 2020; Sokolovsky et al., 2020; Wardell et al., 2024), this was not replicated in all studies (Fairlie et al., 2023), and notably, few of these studies have considered whether differences in level of cannabis consumption may contribute to these findings.
The lack of consistent findings for heightened acute negative consequences on simultaneous use versus single substance use days/events may suggest that elevated negative consequences associated with simultaneous use observed in between‐person studies could be driven by the cumulative effects of simultaneous use on more chronic types of consequences (e.g., dependence symptoms) rather than heightened risk for acute negative outcomes at the event‐level. Further, it may be that unobserved dispositional risk factors associated with propensity to engage in simultaneous use (e.g., impulsivity; Daros et al., 2022; Waddell, Blake, & Chassin, 2021) could account for increased negative consequences among individuals who engage in simultaneous use relative to those who do not; as within‐person analyses are able to better control for person‐level confounds, this could explain divergent findings across within‐person and between‐person studies. Additionally, as only a small proportion of studies reviewed included moderators in their analyses, there may be other variables that influence these relationships and help to explain the discrepancies in results found across studies. For example, one article reported different findings for individuals under age 21 compared with those age 21 and older (Linden‐Carmichael, Allen, & Lanza, 2021), suggesting that age may represent one important moderator. Thus, more research is needed to understand variables that may strengthen the associations between engaging in simultaneous use and greater use‐related consequences.
The finding that consequences are not consistently elevated on simultaneous use occasions compared with single substance occasions, and that level of alcohol consumed may play an important role in consequences on simultaneous use days, could have implications for intervention efforts. Specifically, perhaps interventions should focus on reducing heavy alcohol consumption rather than eliminating simultaneous use altogether. As there is evidence that the quantities of cannabis and alcohol may interact on simultaneous use days to predict the number of negative consequences experienced (Wardell et al., 2024), more work is needed on the role that the forms and amounts of cannabis consumed have in the harms associated with simultaneous use to further inform interventions.
Importantly, there are some methodological factors that may contribute to the mixed findings for differences in the number of consequences across simultaneous use and single substance use days/events. First, there was heterogeneity in how negative consequences were measured, with some studies linking consequences specifically to alcohol or cannabis (i.e., Fairlie et al., 2023; Lee et al., 2020; Waddell, Corbin, et al., 2023) and others assessing non‐substance‐specific consequences (i.e., Boyle et al., 2024; Linden‐Carmichael et al., 2020; Mallett et al., 2019; Sokolovsky et al., 2020; Wardell et al., 2024). Second, there were inconsistencies in study inclusion criteria, as some studies did not specifically recruit individuals who reported recent simultaneous use (i.e., Mallett et al., 2019), and among those that did, there was variability in eligibility requirements regarding drinking behavior and/or simultaneous use behavior (e.g., Fairlie et al., 2023; Lee et al., 2020; Linden‐Carmichael et al., 2020). While all studies recruited young adults, some studies recruited exclusively through college campuses (i.e., Linden‐Carmichael et al., 2020; Mallett et al., 2019; Sokolovsky et al., 2020) and others focused on community samples of young adults (Lee et al., 2020). There was also variation in age ranges included in young adult samples. Finally, among the studies included in the general consequences theme, one study utilized retrospective measures (i.e., TLFB) to assess day‐level use and consequences (Waddell, Corbin, et al., 2023), although findings from this study were similar to studies that utilized daily diary or EMA designs. Thus, future research should aim to establish more consistent methods when examining acute consequences associated with simultaneous use compared with single substance use to help resolve mixed findings in the literature.
Specific consequences
Finally, 14 studies examined specific consequences of use (e.g., sleep, affect, and injury). While the included studies focused on different outcomes, there was a general theme that simultaneous use tends to be associated with greater likelihood of negative consequences relative to single substance use (i.e., self‐harm behavior, experiencing sexual assault, and specific substance‐related consequences such as impaired driving; Fairlie et al., 2023; Patrick et al., 2021; Read et al., 2021; Sellers et al., 2021; Waddell, McDonald, et al., 2023. This review helps to narrow in on the unique consequences associated with engaging in simultaneous versus single substance use, illustrating the importance of examining specific consequences (rather than aggregating heterogeneous consequences into a count variable) to better elucidate the consequences of simultaneous use. However, as each specific substance‐related consequence covered in this review was examined in only one or two studies, further replication is necessary before broader conclusions can be drawn about the impact of simultaneous versus single substance use on specific consequences. One exception is that there was more consistent evidence that simultaneous use was generally associated with better sleep outcomes than alcohol‐only use, but with worse sleep outcomes when compared with cannabis‐only use (Graupensperger et al., 2021; Sznitman et al., 2023, 2024). Together, these studies provide preliminary evidence that combining alcohol with cannabis may offset some of the detrimental effects of alcohol on sleep, but may also negate some of the sleep enhancing effects of cannabis. However, it should be noted that none of the studies examining sleep controlled for quantity of alcohol or cannabis use in their analyses, so there is a need for future studies that account for quantities used and other potential confounds (e.g., social context of use).
There were other important methodological differences across studies of specific consequences. First, two studies utilized retrospective TLFB measures (i.e., Sellers et al., 2021; Waddell, McDonald, et al., 2023), whereas the others used prospective daily diary and EMA designs, but it is difficult to determine whether this contributed to mixed findings given that these studies examined different outcomes. While nearly all articles included young adult samples (except Sellers et al., 2021), even in studies examining similar outcomes, there was heterogeneity across young adult samples (i.e., freshman males vs. young adult females in Brown et al., 2018 and Read et al., 2021 respectively). Thus, there remains both a need for replication efforts of various specific consequences and more consistency in methodological approaches across studies to advance our understanding of specific acute consequences of simultaneous use.
Limitations and future directions
This review highlights many strengths in the extant literature examining within‐person comparisons of simultaneous use to single substance use, notably the large sample sizes and rich data collected through the use of EMA study designs. However, this review also revealed important limitations in the current literature that have not already been mentioned. First, studies nearly exclusively utilized young adult/college student samples. Notably, young adulthood represents a developmental period with elevated rates of substance use that most individuals eventually “mature out” of (Chen & Kandel, 1995; Tucker et al., 2020). Thus, findings from the review may not generalize to other age groups. Similarly, the vast majority of studies took place in the United States, and thus, substance use behaviors and consequences may not be applicable to countries with different cannabis or alcohol policies and norms. Further, several of the papers in the review derived data from the same dataset to answer different research questions; thus, there is a need for future research to replicate findings in unique samples and examine generalizability to broader populations. As only 43% of studies reported that engaging in simultaneous use was an eligibility criterion, findings from the extant literature may be more representative of convenience samples that happen to report simultaneous use events. Moreover, all included studies relied exclusively on self‐report measures. Future research could integrate objective measures (i.e., blood alcohol concentration, location data) into EMA data collection for a more comprehensive understanding of acute outcomes of simultaneous use.
There are also limitations of the current review to consider. First, our review included studies of same‐day co‐use, which introduces some heterogeneity given that simultaneous use is typically defined as using both alcohol and cannabis at the same time with overlapping effects or using both substances within a narrow time window. However, consistent with Lee et al.'s (2022) review, we believed it was important to include studies of same‐day co‐use in order to be more comprehensive in our review given the variability in definitions in the literature. Still, it is important to note that variability in the operationalization of simultaneous use may contribute to mixed findings. Second, this review did not include a meta‐analysis as the antecedents and outcomes across studies were too heterogenous to meaningfully combine. Third, as this review only considered published studies, there may be a file drawer bias that excludes studies with null findings. Finally, this review focused on a specific study design (i.e., within‐person analyses of event/day level associations), so conclusions cannot be generalized to other types of comparisons between simultaneous and single substance use.
CONCLUSIONS
The emerging literature on within‐person differences in proximal antecedents and acute outcomes across simultaneous use and single substance use at the day‐ and event‐level reveals that specific motives and contexts predict simultaneous use relative to single substance use. However, while alcohol consumption was higher on simultaneous use days compared with alcohol‐only days, findings regarding elevated risk for negative consequences on simultaneous use days/events compared with single substance days/events were mixed and may be dependent on level of alcohol consumed. The findings of this review can inform “just in time” interventions by highlighting proximal predictors of simultaneous use that may be promising intervention targets. Additionally, this review helps to identify consequences that are most salient on simultaneous use days to inform targeted treatment and harm reduction efforts among individuals who use both substances.
FUNDING INFORMATION
This work was supported by funding from the Canadian Institutes of Health Research to Jeffrey D. Wardell and Christian S. Hendershot (Grant Number: 159754). During the conduct of this review, Kyra N. Farrelly was supported by a Canada Graduate Scholarship—Doctoral award from the Social Sciences and Humanities Research Council of the Government of Canada.
CONFLICT OF INTEREST STATEMENT
The authors have no conflicts of interest to disclose.
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Farrelly, K.N. , Amini, T. , Coelho, S.G. , Fox, N. , Dimitrova, N. , Hendershot, C.S. et al. (2025) Proximal antecedents and acute outcomes of simultaneous alcohol and cannabis use: Systematic review of daily‐ and event‐level studies. Alcohol: Clinical and Experimental Research, 49, 43–61. Available from: 10.1111/acer.15484
DATA AVAILABILITY STATEMENT
Data sharing is not applicable to this article as no new data were created or analyzed in this study.
REFERENCES
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Associated Data
Data Availability Statement
Data sharing is not applicable to this article as no new data were created or analyzed in this study.