Cannabis use and misuse in the year following recreational cannabis legalization in Canada: A longitudinal observational cohort study of community adults in Ontario

Canada legalized cannabis for non-medical purposes federally in October 2018. This study examined changes in cannabis use over the following year in a sample of general community adults. A secondary aim was to examine forecasted cannabis use following legalization in relation to actual post-legalization cannabis use.

Participants were 1502 community adults (61 % female; age M = 34.60 ± 13.95), with approximately half reporting any cannabis use in the six months prior to legalization (Cannabis+ group [48 %]/Cannabis− group [52 %]). Self-report assessments were conducted in the month before cannabis legalization, 6-months post-legalization and 12-months post-legalization. Primary outcomes were frequency of cannabis use, grams of dried flower cannabis on days used, and level of misuse (Cannabis Use Disorder Identification Test – Revised). Secondary analyses examined pre-legalization personal forecasts in relation to post-legalization cannabis use.

Statistically significant main effects of time (ps<.001), cannabis use status (ps<.001), and time × cannabis use status interactions (ps<.001) were present for cannabis frequency, quantity, and level of misuse. In each case, the interactions reflected significant decreases in the Cannabis+ group, but significant increases in the Cannabis− group. Approximately 15 % of participants erroneously forecasted their personal post-legalization cannabis use, with discrepancies most commonly being individuals who were not using prior to legalization subsequently using cannabis.

In this cohort of community adults, we observed significant changes over the first year following Canadian legalization, with divergent trajectories based on pre-legalization cannabis use. These findings suggest multifarious impacts of legalization in adults, with meaningfully different trajectories among subgroups.