We used data from an established nationally representative Canadian survey to evaluate the relationship between cannabis and codeine use. Overall, 21.6% of Canadians self-reported use of cannabis in 2017, of whom 36.2% used cannabis for medical purposes. Multivariable logistic regression models indicate that individuals who self-reported use of cannabis had greater odds of also reporting use of codeine. When limited to cannabis users, individuals who used cannabis for medical purposes were nearly three times more likely to also report codeine use. These findings reflect the increasing need to further investigate the safety profile of concomitant cannabis and opioid use along with the need for healthcare providers, particularly prescribers (i.e., physicians, nurse practitioners) and pharmacists to assess a patient’s use of cannabis prior to prescribing or dispensing codeine. Furthermore, these findings suggest a potential role for healthcare providers for monitoring patients’ use of cannabis, as the long-term adverse events associated with concurrent use with prescription medications, particularly codeine, remain unknown.

Evidence to support the use of cannabis for treatment of non-cancer chronic pain and opioid dependence has recently emerged, as studies highlight a declining trend in opioid consumption following subsequent cannabis use (Lucas et al. 2021; Dranitsaris et al. 2021). Specifically, a Canadian prospective study (N = 1145) conducted by Lucus et al. reported an overall 78% reduction in mean opioid dosage over the course of 6 months with cannabis use (Lucas et al. 2021). However, it should be noted that study participants received physician guidance regarding appropriate cannabis use throughout the entirety of the study, an aspect which is not observed in the general population (Leos-Toro et al. 2018; Lucas et al. 2021). Additionally, due to the short-term durations (e.g., 6 months) of studies assessing the impact of cannabis on opioid consumption, many fail to assess the potential for long-term adverse effects. Furthermore, with concomitant use, it is important to consider harms associated with polypharmacy, which has always been shown to be more significant than single drug use (Lake et al. 2020; Giummarra et al. 2015).

Our study adds to the existing body of literature by highlighting the strong positive association between the use of cannabis and codeine, two substances which are widely accessible to patients without prescription and/or healthcare professional supervision, particularly in recent years in Canada (Novak et al. 2016). To contextualize with studies in other jurisdictions, in 2018, Caputi et al. used nationally representative individual-level data and reported positive association (aOR 1.66 95% CI 1.49 to 1.83) between use of cannabis and pain relieving medications (Caputi and Humphreys 2018), similar to our current study. Additionally, our study results reflect a strong association (3-fold odds) between the use of cannabis for medical purposes and codeine use. While our study was unable to specify which medical condition study participants were using medical cannabis for, this substantial increase in odds of codeine use among individuals who used cannabis for medical purposes may be attributed to experiencing greater pain severity as a majority of people who use cannabis for medical purposes do so for pain relief (Lake et al. 2019; Boehnke et al. 2022; Leung et al. 2022).

In addition to findings on the association between cannabis and opioid use, our findings also highlight that people who report very good/excellent perceived general health were less likely to report codeine use, compared to those who perceive their health as fair/poor. These findings are in line with previous literature demonstrating poorer physical and mental health scores among individuals who report opioid use (Millson et al. 2004). Another unique finding of our study was that among cannabis users, males were less likely to use codeine in comparison with females. These findings are in line with previous literature that highlights differences in opioid prescription rates among males and females. In comparison to males, females are more likely to be prescribed opioids (Serdarevic et al. 2017), which may be explained in part due to the increased prevalence of chronic conditions resulting in pain among females, such as inflammatory arthritis, fibromyalgia, and migraines (Darnall et al. 2012).

Limitations must be considered when interpreting the results of this study. First, the CTADS survey was conducted between February and December of 2017; therefore, the survey was conducted prior to legalization of cannabis across Canada (October 17, 2018) which may put study results at risk of respondent or social desirability bias. However, it should be noted that access to cannabis for medical purpose has been legalized in Canada since 2000. While we do acknowledge the rising prevalence of cannabis use since legalization in 2018 (Rotermann 2021), the intent of our work was not to establish cannabis use patterns but rather to explore the potential for concurrent cannabis and codeine use. Future research is required to further explore the relationship and impacts of concurrent cannabis and opioid use. Second, our study is restricted to the variables included in the CTADS survey, which does not include information on how frequently participants consumed codeine-containing product(s), limiting our results from truly assessing concurrent use. Third, the cross-sectional study design precludes the ability to infer temporality between exposure and outcome variables. Fourth, exclusion of survey participants with invalid responses to explanatory, outcome, and confounding variables may have impacted the external validity of our study, as a greater proportion of participants included in our study received education past secondary school in comparison with those excluded, which may bias the result of our analysis. Fourth, we excluded survey participants (n = 890) with invalid responses to explanatory, outcome, and confounding variables. Comparison with our study sample on sociodemographic characteristics (i.e., sex, age, geographic region, and education) revealed no differences except with respect to education, whereby a lower proportion of excluded participants reported completing secondary school education or did not provide a response to the question. Lastly, due to sample size limitations, certain descriptive variables did not meet Statistics Canada’s definition for acceptable “quality,” as the coefficient of variation was above their stated threshold (Statistics Canada 2021). Therefore, we were unable to include covariates that have been shown to be associated with codeine use such as use of psychoactive substances, cannabis use characteristics (i.e., method, frequency, self-perceived impairment, and reason for medical cannabis use), and alcohol consumption to maintain the quality of our analysis.