The proportion of schizophrenia cases linked to cannabis use disorder increased nearly threefold over time, with young males experiencing the highest attributable risk, according to a recent study.
In the study, published in JAMA Network Open, investigators from the Ottawa Hospital Research Institute and affiliated institutions conducted a population-based cohort study to examine changes in the population-attributable fraction (PARF) of schizophrenia associated with cannabis use disorder (CUD) following medical cannabis liberalization and nonmedical cannabis legalization in Canada. The investigators analyzed data from 13.6 million participants aged 14 to 65 years in Ontario between January 1, 2006, and December 31, 2022. The participants with a prior schizophrenia diagnosis were excluded. The investigators defined CUD based on emergency department or hospital diagnoses using ICD-10-CA codes, while schizophrenia cases were identified through a validated medical records algorithm. They then assessed trends across three policy periods: prelegalization (January 2006 to November 2015), medical and nonmedical cannabis liberalization (December 2015 to September 2018), and nonmedical cannabis legalization (October 2018 to December 2022).
The findings indicated that the PARF of CUD-associated schizophrenia increased nearly threefold, from 3.7% (95% confidence interval [CI] = 2.7%–4.7%) during the prelegalization period to 10.3% (95% CI = 8.9%–11.7%) and were lowest among female participants aged 45 to 65 years (PARF = 1.8%, 95% CI = 1.1%–2.6%). Over the study period, 91,106 participants (0.7%) developed schizophrenia. A higher incidence was observed in those with CUD (8.9% vs 0.6% in the general population). The annual incidence of schizophrenia remained stable, but the incidence of psychosis not otherwise specified (NOS) increased 83.7% from 30.0 to 55.1 per 100,000 participants. The investigators noted that the increase in the PARF for schizophrenia predated cannabis policy changes, with no significant acceleration following legalization. However, the PARF for CUD-associated psychosis NOS increased more rapidly following medical cannabis liberalization.
The findings suggested that the association between CUD and schizophrenia has strengthened over time, particularly among younger male participants, and may warrant continued monitoring. While the study controlled for multiple confounders, limitations included potential residual confounding and evolving diagnostic criteria for psychotic disorders. Further research is needed to assess the long-term impact of cannabis commercialization on psychiatric outcomes.
Full disclosures can be found in the published study.
