First-episode psychosis and substance use involvement across birth cohorts in Canada: a retrospective longitudinal population-based study

Background — Substance use has rapidly increased in North America. While acute harms (eg, overdoses) are well documented, less is known about potential other harms, including psychotic disorders that are associated with substance use. This study aimed to examine changes by birth cohort in first-episode psychosis incidence and substance use involvement in first-episode psychosis.

Methods — In this population-based study using health administrative data from Ontario, Canada, we included all individuals aged 14–50 years, without a history of psychosis, between Jan 1, 2006, and Dec 31, 2023. We identified: first, all new diagnoses of cases of first-episode psychosis (including substance-induced psychosis, schizophrenia spectrum disorders, or psychosis not otherwise specified); and second, substance-induced psychosis. The primary outcomes were incidence rates and cumulative incidence of first-episode psychosis. Secondary outcomes were the proportion of first-episode psychosis diagnoses in which individuals had hospital-based care for substance use in the preceding 3 years, and the proportion of first-episode psychosis cases coded as substance-induced. We compared outcomes by birth cohorts in 5-year increments between 1970 and 2004 using Poisson models. People with related lived experience were not involved in the research and ethnicity data were not available.

Findings — We included 7 451 463 individuals (mean age 28·8 years [SD 9·7]; 50·2% male and 49·8% female), of whom 114 423 (1·5%) were diagnosed with first-episode psychosis. Diagnoses of first-episode psychosis and substance-induced psychosis increased over time and by birth cohort. In individuals aged 20–24 years, incidence of first-episode psychosis increased by 24·9% (from 138·0 to 172·3 per 100 000 individuals) and of for substance-induced psychosis by 114·1% (from 17·0 to 36·4 per 100 000 individuals) between 2006 and 2023. Compared with those born in 1980–84, by age 30 years, individuals born in 1990–94 had approximately double (adjusted rate ratio [aRR] 2·54 [95% CI 2·46–2·61]) the cumulative incidence of first-episode psychosis (1·85% vs 0·73%) and quadruple (aRR 4·85 [95% CI 4·45–5·28]) the cumulative incidence of substance-induced psychosis (0·36% vs 0·08%). Substance involvement in first-episode psychosis rose by birth cohort. For example, in individuals aged 25–29 years with first-episode psychosis, the proportion with previous substance use care increased from 26·4% to 47·1% (aRR 1·76 [95% CI 1·65–1·87]) and the proportion of first-episode psychosis coded as substance-induced increased from 8·7% to 20·0% (aRR 2·24 [95% CI 1·98–2·54]) for those born in 1995–99 relative to 1980–84. Increases in cannabis involvement in first-episode psychosis were greater than increases associated with other substances over the study period.

Interpretation — The findings suggest that rising substance use could be associated with a higher prevalence and earlier age of onset of psychotic disorders in more recent birth cohorts. Upstream prevention strategies to reduce substance use, and improved clinical capacity for psychosis care, are indicated.