ImportanceA 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.ObjectiveTo 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.DesignProspective cohort study with 11 biannual assessments from September 2018 to October 2023. Mean retention was 90% across all waves.SettingOntario, Canada.ParticipantsSample of 1,428 (60.2% female, Mage=34.5) community adults aged 18 to 65 years.ExposureFive years of recreational cannabis legalization (the baseline wave was immediately prior to legalization).Main outcome and measuresPrimary 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.ResultsLinear 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 RelevanceIn 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.Key pointsQuestionDid cannabis use or misuse change among adults in the five years following recreational cannabis legalization in Canada (overall and by pre-legalization cannabis use frequency)?FindingsOverall, cannabis use frequency increased significantly while misuse decreased significantly, with small effect sizes for both. Pre-legalization cannabis use significantly moderated these changes. Product preferences shifted from dried flower, hashish, and concentrates to edibles, liquids, and vape pens.MeaningFrom a public health standpoint, these findings suggest both a modest negative consequence (small increase in cannabis use frequency) and positive outcomes (small decrease in cannabis misuse, and transition from combustible to non-combustible products).
