Cannabis Recreational Legalization and Prevalence of Simultaneous Cannabis and Alcohol Use in the United States

Background: Simultaneous cannabis/alcohol use, using both substances within a short time interval so that their effects overlap, has a greater risk of potential negative consequences than single-substance use and is more common in younger age. Relationships between recreational cannabis laws (RCLs) and changes in simultaneous cannabis/alcohol use prevalence remain untested. Objective: To examine trends in simultaneous cannabis/alcohol use from 2008 to 2019, and investigate associations between implementation of RCLs (i.e., presence of active legal dispensaries or legal home cultivation) and simultaneous cannabis/alcohol use in the United States (U.S.). Design: Repeated cross-sectional samples from the 2008–2019 U.S. National Survey on Drug Use and Health (NSDUH). Participants: Respondents (51% female) aged 12 and older. Interventions: Changes in simultaneous cannabis/alcohol use before and after RCL implementation (controlling for medical cannabis law implementation) were compared in different age groups (12–20, 21–30, 31–40, 41–50, 51+), using adjusted multi-level logistic regression with state random intercepts and an RCL/age group interaction. Measurements: Self-reported simultaneous cannabis/alcohol use. Results: From 2008 to 2019, the overall prevalence of simultaneous cannabis/alcohol use declined among those aged 12–20 but increased in adults aged 21+. Model-based simultaneous cannabis/alcohol use prevalence increased after RCL implementation among respondents aged 21–30 years (+1.2%; aOR= 1.15 [95%CI = 1.04–1.27]), 31–40 years (+1.0; 1.15 [1.04–1.27]), and 41–50 years (+1.75; 1.63 [1.34–1.98]), but not in individuals aged <21 or 51+ years. Conclusions: Implementation of recreational cannabis policies resulted in increased simultaneous use of cannabis and alcohol, supporting the complementarity hypothesis, but only among adults aged 21+. Efforts to minimize harms related to simultaneous cannabis/alcohol use are critical, especially in states with RCLs. Future studies should investigate cultural norms, perceived harm, and motives related to simultaneous use.