Cannabis Use and Misuse Following Recreational Cannabis Legalization.

Importance: An increasing 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 5 years following legalization in Canada both overall and by prelegalization cannabis use frequency using a longitudinal design. Design, Setting, and Participants: This prospective cohort study included data from community-dwelling adults who participated in up to 11 biannual assessments from September 2018 to October 2023 in Ontario, Canada. Data were analyzed from November 2023 to January 2024. Exposure: Five years of recreational cannabis legalization (baseline wave was immediately prior to legalization). Main Outcome and Measures: Primary outcomes were cannabis use frequency and cannabis misuse, assessed using Cannabis Use Disorder Identification Test - Revised (CUDIT-R) score. Prelegalization cannabis use frequency, age, and sex were examined as moderators. Secondary outcomes included changes in cannabis product preferences over time. Results: The final cohort included 1428 community-dwelling adults aged 18 to 65 years (859 [60.2%] female; mean [SD] age, 34.5 [13.9] years). Mean retention was 90% across all waves. Linear mixed-effects modeling found a significant increase in cannabis use frequency, such that the mean proportion of days using cannabis increased by 0.35% (95% CI, 0.19% to 0.51%) per year (P < .001) in the overall sample (1.75% over 5 years). In contrast, CUDIT-R scores (on scale of 0 to 32) decreased significantly overall (β = -0.08 [95% CI, -0.10 to -0.06] per year; -0.4 over 5 years; P < .001), most notably with the onset of the COVID-19 pandemic. Interaction analyses indicated that prelegalization cannabis use frequency significantly moderated changes for both outcomes (P < .001). Specifically, cannabis use and misuse decreased among prelegalization frequent consumers and modestly increased among occasional users and nonusers. Cannabis product preferences shifted away from dried flower, hashish, concentrates, oil, tinctures, and topicals to edibles, liquids, and vape pens. Conclusions and Relevance: In this prospective cohort study of community-dwelling adults in Canada, cannabis use frequency increased modestly in the 5 years following legalization, while cannabis misuse decreased modestly. These changes were substantially moderated by prelegalization cannabis use, with more frequent consumers of cannabis before 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 and positive consequences among adults.