Cannabis use has been highly prevalent among COMPASS youth since 2012/2013, as it is among their peers across Canada.7 27 28 Our data generally suggest that, following a steady decrease over several years, there has been a gradual increase in youth cannabis use since the beginning of the federal discourse around legalisation. During this time, public perception of cannabis use shifted to include a type of ‘sensible’ use, which is intermittent and socially acceptable.32–35 For youth, these more accommodating social norms and pro-cannabis messaging are associated with increased cannabis use.36–38 Here, most of the increase we observed was due to an upswing of occasional, or sensible, use. When placed in the context of growing access after 2014/2015 and the slow rise in average age at first use, these data may represent the emergence of a large group of casual cannabis users who start later and use infrequently. Future, postlegalisation evaluations must therefore explore the impact on various patterns of use.

We may characterise our results further by relying on the normalisation theory framework, which posits that the interplay between six factors—access, trying and usage rates, attitudes, cultural accommodation and governmental response—can normalise drug use.39 40 Other factors have recently been suggested, for example, seeing drugs as a means of achieving normal goals, such as cannabis use by athletes to enhance performance.41–43 Indicators for the normalisation of cannabis use have been prevalent in Canada for some time.34 44–46 Subsequent to reduced restriction of medical cannabis in 2014, rates of students who thought cannabis easy to access significantly increased in 2015/2016 (Y4) and rose in tandem with further deregulation.1 2 The increased availability of cannabis is likely to have normalised and facilitated access for youth, who most commonly obtain drugs through their social network.40 47 Perceived access drives use48 and we identified that rates of cannabis use in our sample increased the following year (2016/2017). This is true especially among female students, for whom the effects of normalisation may be greater as their cannabis use has historically been more stigmatised than that of their male peers.49 Female students are also more likely to use alternative cannabis products, which were then becoming more accessible.2 50 51 As normalisation and access to diverse cannabis products is liable to increase in Canada after legalisation,19 22 special attention should be paid to subsequent impacts on female youth.

The evidence presented here demonstrates that the focus on demographic differences among youth who use cannabis is warranted. For instance, lifetime and past-year use rates were particularly high among Aboriginal students, who also reported the most weekly use. This is in line with existing evidence reporting similar results for both American and Canadian Aboriginal youth, who are more likely to start younger and use cannabis more regularly than white adolescents, especially if male.52–57 Following a steady decrease over several years, weekly use by Aboriginal youth increased significantly in 2017/2018 (Y6). This is a concerning development that may warrant close future monitoring, and interventions to mitigate the effects of cannabis legalisation for these adolescents are likely to be needed.

The evidence also suggests that studies on the impact of cannabis legalisation on Canadian youth should consider data from several years prior to the passage of the Cannabis Act. Relying on 2017/2018 as a solitary baseline is liable to miss key developments in the years before, and therefore likely to underestimate subsequent effect, for example, the increase in youth lifetime and past-year use. In 2016/2017, Health Canada funded the expansion of COMPASS to include high school students in Québec, British Columbia, and Nunavut, increasing the relevance of its future investigations of youth cannabis use. As cannabis legalisation is likely to disproportionally affect youth, these data will be essential to understand and mitigate its impact.