Cannabis Use Did Not Rise Broadly Among High-Risk Young Adults After Canada’s Legalization

A longitudinal study of young adults in Ontario found no broad increase in cannabis use or cannabis-related problems after Canada legalized nonmedical cannabis in 2018. Instead, use and reported consequences declined overall, although the pattern differed depending on participants’ cannabis use before legalization.

The study, published in JAMA Network Open in 2023, followed 619 people aged 19.5 to 23 who had reported at least two heavy episodic drinking occasions. Researchers surveyed them every four months for three years, covering two prelegalization periods, the transition to legalization, and four postlegalization follow-ups.

Canada’s Cannabis Act took effect on October 17, 2018. The law established a regulated system for producing, distributing, selling and possessing cannabis for adult use.

The participants were not a representative sample of all Canadian young adults. They were recruited because their existing pattern of alcohol use placed them at elevated risk for substance-related harms. Before legalization, 14.5% reported using cannabis daily or multiple times a day, while 17.1% reported weekly use. Another 33.3% used it monthly, 22.6% were not currently using it, and 12.4% had never used it.

Across the full sample, average cannabis-use frequency declined modestly over the seven assessment periods. Cannabis-related consequences—measured with a questionnaire covering problems associated with use—also decreased, with the decline statistically significant during the postlegalization periods.

The largest reductions occurred among participants who had used cannabis most frequently before legalization. Their cannabis use and reported consequences fell after legalization, a pattern the researchers said was consistent with an “aging out” trajectory often seen as young adults move beyond earlier patterns of substance use.

Participants who were not currently using cannabis before legalization showed the opposite pattern: their use increased modestly over time. However, the increase began before legalization and was not accompanied by a significant rise in cannabis-related consequences. Those who had never used cannabis before legalization did not show significant postlegalization increases in either use or consequences.

These findings do not demonstrate that legalization caused cannabis use to decline. The study did not include a comparable control group in a jurisdiction where cannabis remained illegal, so it cannot establish what would have happened without the policy change. The authors also cautioned that the sample was nonrepresentative, included young adults already engaging in risky alcohol use, and covered only the early years of legalization. The study examined the initial legalization of dried cannabis; legal sales of edibles and concentrates expanded later.

The results therefore offer a narrower conclusion than the claim that legalization is harmless or that cannabis use falls whenever prohibition ends. They suggest that, in this particular high-risk group, legalization was not followed by a substantial near-term surge in consumption or cannabis-related problems. Canada’s broader surveillance efforts, including the annual Canadian Cannabis Survey, remain important for tracking changes across different age groups and patterns of use.

Nor does the study eliminate concerns about cannabis use during adolescence or young adulthood. Public-health agencies continue to warn that frequent or heavy use can affect attention, memory, coordination and other aspects of a developing brain. The study’s main contribution is more specific: among these Ontario young adults, the transition to legal adult cannabis sales did not produce the large, generalized increase in use or consequences that some critics had predicted.

The authors called for continued longitudinal research to distinguish the effects of legalization from normal changes in substance use as people age and to identify whether longer-term effects emerge in groups not represented by this cohort.