BACKGROUND AND AIMS: Epidemiologic research suggests that adolescent cannabis use is associated with psychological distress (i.e. depression and anxiety symptoms); however, most studies have relied on 20th-century data, when cannabis was significantly less potent than today. This study aimed to estimate the association between adolescent cannabis use and psychological distress using contemporary population-based data and examine the roles of time [as a proxy for increasing Δ9-tetrahydrocannabinol (THC) potency], sex and age of initiation.

DESIGN: Representative cross-sectional survey conducted biennially from 2013 to 2023.

SETTING: Ontario, Canada.

PARTICIPANTS: 35 007 adolescents in grades 7 to 12.

MEASUREMENTS: Past-year cannabis use was categorized as Never, 1-2 times, 3-9 times, 10-39 times or 40+ times. Psychological distress was measured with the Kessler-6 scale using a cut-off score of 13+ indicating anxiety/depression symptoms. Multivariable modified Poisson and least-squares models were used to estimate the association between past-year cannabis use and psychological distress. Survey year and sex were tested as effect modifiers on the multiplicative and additive scales. The association between school grade of cannabis use initiation and psychological distress was also estimated.

FINDINGS: From 2013 to 2023, the prevalence of psychological distress increased from 10.7% to 27.4%, whereas cannabis use decreased from 23.1% to 17.6%. Survey year and sex were statistically significant effect modifiers for the association between cannabis use and psychological distress with associations consistent with a super-additive effect but not multiplicative synergy (additive interactions: P < 0.05; multiplicative interactions: P > 0.05). The association between cannabis use and psychological distress strengthened over time, particularly for those using 40+ times compared with abstinence (from 0% [95% confidence interval (CI) = -6% to 6%] adjusted prevalence difference in 2013 to 18% (95% CI = 11%-25%] adjusted prevalence difference in 2023). Independent of time, there was evidence of dose-response among females, but not males. A 5% (95% CI = 1%-10%) lower prevalence of psychological distress was observed per later school grade of cannabis use initiation.

CONCLUSIONS: Psychological distress increased markedly among adolescents in Ontario, Canada, from 2013 to 2023. In that setting, adolescent cannabis use was statistically significantly associated with psychological distress, especially among females, and this association increased in magnitude over time, especially for those using most frequently. It is possible that adolescents are increasingly self-medicating psychological distress with cannabis and/or that rising cannabis potency is increasingly contributing to psychological distress. While causality cannot be established, based on the precautionary principle, policymakers should prioritize cannabis prevention strategies that aim to reduce frequency of use, limit potency and delay age of initiation, particularly among females.