Despite the prevalence of cannabis and cigarette co‐use and emerging evidence of interactive effects of these substances on the brain, the impact of cigarette co‐use on cannabis cue reactivity—an important biomarker of CUD—has not previously been investigated. As such, the goal of the current study was twofold: first, to replicate previous findings showing heightened neural cannabis cue reactivity in heavy users compared with controls using groups closely matched on cigarette use; second, to investigate potential differences in neural cannabis cue reactivity between co‐users and cannabis‐only users. In contrast to our expectations, we did not find elevated cannabis cue‐specific brain activity in our five ROIs or in the exploratory whole brain analysis when we combined cigarette and non‐cigarette smoking cannabis users compared with a sample of matched noncannabis using controls. This differs from previous findings of heightened activity to cannabis cues in frequent (at least 10 uses per month), daily, and dependent (meet DSM‐IV criteria for CUD) cannabis users regardless of cigarette use status in the VTA, ACC, amygdala, striatum, and frontal cortical regions. ¹⁹ , ²² , ²⁴ When cannabis users and controls were split based on cigarette use, we observed heightened cannabis cue‐induced activity in the amygdala in the cannabis‐only users compared with non‐cigarette smoking controls, but not in co‐users. In co‐users, cannabis cue reactivity in the VTA was associated with severity of use, but not in the other ROIs. Unexpectedly, no associations between cannabis use measures and brain activity were observed in cannabis‐only users. These results show that cigarette use matters when studying cannabis cue reactivity and should be considered in future research and interpretations of previous studies.

One explanation for the limited cannabis cue‐specific activity observed in cannabis‐only users—and not in co‐users—may be the inclusion of a community sample of heavy users with varying degrees of problematic cannabis use, as cannabis cue reactivity increases with severity of use. ¹⁹ The average score on the CUDIT‐R in this sample was 13.5, which is above the suggested cutoff of 13 indicative of problematic and potentially dependent use. ³¹ However, scores ranged from 3 to 24 and 41% of the sample fell below the cutoff point, which may explain the limited cannabis cue reactivity observed when averaged across the group. The consistent cannabis–cigarette use interactions we observed across multiple ROIs indicate that cigarette use in both cannabis users and controls matters when assessing cannabis cue reactivity. These findings suggest that combining co‐users and cannabis‐only users into a single group may diminish the ability to detect cannabis‐specific cue reactivity in the brain. In support of this hypothesis, in previous research, the most widespread cannabis cue‐induced activation patterns emerged across the frontal, cingulate, and midbrain (e.g., VTA) areas when the proportion of co‐users in the sample was low (i.e., Filbey et al ²² ; average of 1.5 cigarette smoking days in past 60 days). In studies that had high proportions of co‐use in nondependent heavy users (58% in Zhou et al ¹⁹ and 68% in Cousijn et al ²⁴ ), cannabis cue‐induced activity was observed in much narrower circuits including the VTA, medial prefrontal cortex, and superior parietal areas.

The mechanism underlying a potential reduced or lack of cannabis neural cue reactivity in co‐users remains unclear. Preliminary evidence suggests that co‐occurring use of tobacco products may have a “masking” effect on cannabis‐induced alterations in cognition and brain function. However, very little research has been done in this area, with the only evidence in humans coming from studies of functional connectivity and memory performance. In the studies of functional connectivity, co‐users differed from cannabis and tobacco only users, but these differences were not clearly additive, making interpretation challenging. ⁴ , ²⁵ The clearest evidence for a masking effect of tobacco use is from memory performance during acute intoxication, where co‐administration inhibited the impairment of cannabis administration alone on delayed memory recall. ⁶ Regardless of the underlying mechanism, an absence of cue reactivity in co‐users has potentially important methodological implications for cannabis cue exposure research in terms of study design and user group characteristics. It is common for studies investigating heavy use and CUD to match users and controls on cigarettes use. Although this theoretically prevents cigarette use from confounding results, our results suggest that large proportions of cigarette smokers in cannabis users and control groups may in fact obscure cannabis‐specific reactivity. Given the limited evidence base on the effects of co‐use, our findings provide preliminary evidence for differences between cannabis‐only and co‐users on an important biomarker of CUD. It is necessary to replicate these findings in a clinical sample with CUD, as cannabis cue reactivity increases with severity of use‐related problems and individuals who seek treatment may differ from those who do not. ¹⁹ Future studies should aim to determine the potential clinical implications of these findings. Based on our results, it may be that cue reactivity is a more important factor in the development and maintenance of CUD in cannabis‐only users. If this is the case, therapies that involve cue exposure may be less effective in individuals with CUD who also regularly smoke cigarettes. Furthermore, a next step would be to investigate whether specific patterns of co‐use of cannabis and cigarettes (or other tobacco products) differentially influence cannabis cue reactivity. For instance, does it matter whether the drug effects overlap? How important is the route of administration in the potential masking effect of cigarette use? Would we see reduced cannabis cue reactivity in individuals who smoke joints but vape nicotine products? Ecological momentary assessment may be a useful tool to clarify whether daily patterns of use (e.g., used in conjunction in the case of a spliff or used separately in the day) relate to alterations in cannabis cue reactivity and what mechanisms may underlie these alterations.

Interestingly, our results also revealed a consistent pattern of cannabis–cigarette use interactions driven by unexpectedly elevated cannabis cue‐specific activity in cigarette smoking controls. That is, relative to the cannabis users, cigarette using controls showed elevated cannabis cue‐specific activity in bilateral amygdala, striatum and ACC, and in the left frontal pole and inferior frontal gyrus (IFG). But why would cigarette smokers show elevated cannabis cue reactivity? A tempting but tentative explanation could lie in a “tobacco gateway effect.” Previous research indicates that adolescents who currently or formerly use tobacco are more likely to initiate cannabis use and are at higher risk of developing cannabis dependence. ⁴⁵ , ⁴⁶ , ⁴⁷ Post hoc correlational analysis suggests that our effects are independent of the severity of cigarette use (nonsignificant Pearson correlation coefficients ranging from r = −.159 to .233 in the ROIs). Tobacco use may sensitize individuals to cannabis cues regardless of severity of use, which might give insight into a potential neurocognitive mechanism of the gateway from cigarette to cannabis use. If this hypothesis is true, cannabis cue reactivity should predict future cannabis use in tobacco users. Future research should aim to replicate our coincidental finding of heightened cannabis cue reactivity in cigarette smokers in light of the gateway hypothesis with a larger sample and a longitudinal design to test the predictive value of neural cannabis cue reactivity in the initiation of cannabis use in smokers. Alternatively, it is possible that the incorporation of cigarette cues in the cue reactivity task may have caused diffused cigarette craving across conditions. However, in the amygdala, striatum, and ACC, we consistently found that cigarette smoking controls had higher activity that co‐users. If the cigarette cues were causing diffuse cigarette craving, we would expect co‐users to also show this effect.

A strength of the current study is the use of closely matched groups on key demographic characteristics, drug use, and mental health variables, which allowed us to examine cannabis‐ and cigarette‐specific effects. AUDIT score was the only matching variable in which any group difference was observed; cigarette smokers had significantly higher alcohol use and related problems than nonsmokers. Because we did not observe main effects of cigarette use on our outcome variables of interest, it is unlikely that this difference confounded our results although it cannot be ruled out entirely. A further strength is the systematic assessment of the effect of cigarette use on cannabis cue reactivity. First, by recruiting closely matched subgroups of cigarette smokers and nonsmokers in both the cannabis users and nonusing controls, we were able to examine interactive effects of cigarette and cannabis use. Second, by incorporating cigarette cues in the cue reactivity paradigm, we were able to isolate cannabis‐induced activity using a strict (Cannabis > Neutral + Cigarette) contrast. Given the similarity in the route of administration, cannabis cues (e.g., someone smoking a joint) may unintentionally activate cigarette‐related activity in the brain. By subtracting out all activity observed during the presentation of cigarette cues, we were able to isolate cannabis‐specific activity.

A limitation of the current study is the small sample size once the cannabis and control groups were split on cigarette use. Given the general pattern of reduced neural cue reactivity in co‐users compared with cannabis‐only users, future studies investigating cannabis–cigarette interactions on cue reactivity and cannabis‐specific effects in general should aim to use larger samples in order to be sufficiently powered to detect small to medium effect sizes. In addition, a limitation of the current cue exposure task was the lack of “online” assessments of craving during the task. It is therefore advised to incorporate in‐task assessments of craving after cue presentation (as in Filbey et al ¹⁷ , ²² cue reactivity paradigm) in order to further delineate between cannabis and cigarette craving. Furthermore, the influence of subacute intoxication on neurocognitive responses is a significant issue cutting across research on current heavy cannabis users. We were limited in our ability to control for potential residual subacute effects of intoxication, a salient issue given evidence that acute THC intoxication is associated with reduced striatal responding. ⁴⁸ Investigating the associations between residual concentrations of drug metabolites and neural responses could garner important insight into potential mechanisms underlying differences between cannabis‐only and co‐users.

In conclusion, our findings offer preliminary evidence that cigarette use matters when measuring neural cannabis cue reactivity. Cannabis‐only users showed heightened cannabis cue reactivity in the amygdala, but co‐users did not in any ROI. These findings underscore the importance of considering cigarette smoking status when investigating the role of cue reactivity in heavy cannabis use, especially in the context of sample composition. Even when user and control groups are matched on cigarette use, co‐use may obscure specific effects of cannabis in single‐substance users. Furthermore, given the high prevalence of co‐use, ² it is crucial for future studies to specifically investigate differences between cannabis‐only and co‐users in the underlying mechanisms of CUD.