In Ontario, the proportion of new schizophrenia cases attributable to cannabis use disorder rose from 3.7% before legalization to 10.3% after, reaching 18.9% among young men aged 19-24.
Policymakers evaluating cannabis legalization impacts, psychiatrists, public health researchers studying psychosis epidemiology.
18.9% of new schizophrenia cases in young men (19-24) were associated with cannabis use disorder post-legalization
What the researchers found
The population-attributable risk fraction (PARF) for CUD associated with schizophrenia nearly tripled from 3.7% pre-legalization to 10.3% post-legalization. Among males aged 19-24, the PARF reached 18.9%. The overall incidence of schizophrenia remained stable, but psychosis NOS increased 83.7% post-legalization. The PARF increase was steady over time without acceleration after specific policy changes.
Why it matters
This is among the most comprehensive population-level studies linking cannabis policy changes to psychotic disorder burden. The finding that nearly 1 in 5 schizophrenia cases in young men may be attributable to CUD post-legalization carries significant public health implications.
The numbers in context
13,588,681 individuals; 118,650 (0.9%) had CUD; 91,106 (0.7%) developed schizophrenia; PARF rose from 3.7% to 10.3%; PARF for males 19-24 reached 18.9%; PARF for females 45-65 was 1.8%; psychosis NOS incidence rose from 30.0 to 55.1 per 100,000 (83.7% increase).
How the study worked
Population-based cohort study of 13.6 million Ontario residents aged 14-65 without prior schizophrenia history, from 2006-2022, using segmented linear regression to examine changes across three policy periods: pre-legalization, medical liberalization, and non-medical legalization.
What this study cannot tell us
Observational design cannot prove that CUD causes schizophrenia. The PARF increase was gradual and did not accelerate after specific policy changes, making it difficult to attribute directly to legalization versus other temporal trends. Reverse causation (psychosis prodrome leading to cannabis use) remains possible.
How to read the evidence
Strong: Very large population-based cohort (13.6 million) published in JAMA Network Open with rigorous time-series analysis across three policy periods.
When this study was published
Published in 2025 with data from 2006-2022.
The bigger picture
This study adds to a growing body of evidence connecting cannabis legalization, rising CUD rates, and psychotic disorder burden. The stable overall schizophrenia incidence alongside rising psychosis NOS suggests the diagnostic landscape may be shifting as cannabis-related presentations increase.
Questions still open
- Why did the PARF increase steadily rather than accelerating after legalization? Is the rising psychosis NOS incidence a precursor to future schizophrenia diagnoses? What specific components of cannabis policy (potency limits, advertising, access) could mitigate these trends?
Common questions
Does cannabis cause schizophrenia?
Why did the trend not accelerate after legalization?
Read the original research
Changes in Incident Schizophrenia Diagnoses Associated With Cannabis Use Disorder After Cannabis Legalization.
JAMA network open, 8(2), e2457868
Citation
Myran, Daniel T; Pugliese, Michael; Harrison, Lyndsay D; Solmi, Marco; Anderson, Kelly K; Fiedorowicz, Jess G; Finkelstein, Yaron; Manuel, Doug; Taljaard, Monica; Webber, Colleen; Tanuseputro, Peter. (2025). Changes in Incident Schizophrenia Diagnoses Associated With Cannabis Use Disorder After Cannabis Legalization.. JAMA network open, 8(2), e2457868. https://doi.org/10.1001/jamanetworkopen.2024.57868
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