Longitudinal changes in cannabis use and misuse in the 5 years following recreational cannabis legalization in Canada: A prospective cohort study of community adults

Importance: A growing number of jurisdictions have legalized recreational cannabis for adults, but most evaluations have used repeated cross-sectional designs, preventing examination of within-person and subgroup trajectories across legalization. Objective: To examine changes in cannabis use and misuse in the five years following legalization in Canada - the first G7 country to legalize adult recreational cannabis use - both overall and by pre-legalization cannabis use frequency using a longitudinal design. Design: Prospective cohort study with 11 biannual assessments from September 2018 to October 2023. Mean retention was 90% across all waves. Setting: Ontario, Canada. Participants: Sample of 1,428 (60.2% female, Mage=34.5) community adults aged 18 to 65 years. Exposure: Five years of recreational cannabis legalization (the baseline wave was immediately prior to legalization). Main outcome and measures: Primary outcomes were cannabis use frequency and cannabis misuse (CUDIT-R score). Pre-legalization cannabis use frequency, age, and sex were examined as moderators. Secondary outcomes included changes in cannabis product preferences over time. Results: Linear mixed effects modelling found a significant increase in cannabis use frequency, such that the mean proportion of days using cannabis increased by 0.35% (p<.001) per year in the overall sample (1.75% over 5 years). In contrast, CUDIT-R scores (on scale of 0 to 32) decreased significantly overall (b=-0.08 [-0.4 over 5 years], p<.001), most notably following the onset of the COVID-19 pandemic. Interaction analyses indicated that pre-legalization cannabis use frequency significantly moderated changes for both outcomes (p<.001). Specifically, cannabis use and misuse decreased among pre-legalization frequent consumers and modestly increased among occasional/non-users. Cannabis product preferences shifted away from dried flower, hashish, concentrates, oil, tinctures, and topicals to edibles, liquids, and vape pens. Conclusions and Relevance: In the five years following legalization, cannabis use frequency increased modestly, while cannabis misuse decreased modestly in a longitudinal observational cohort of Canadian adults. These changes were substantially moderated by pre-legalization cannabis use, with more frequent consumers of cannabis pre-legalization exhibiting the largest decreases in both outcomes. Although longer-term surveillance is required, these results suggest Canadian recreational cannabis legalization was associated with modest negative consequences and some evidence of positive outcomes among nonclinical community adults.