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The proportion of schizophrenia cases linked to cannabis use disorder in Ontario nearly tripled from 2006 to 2022 amid major cannabis policy liberalization.

The proportion of new schizophrenia cases associated with cannabis use disorder (CUD) nearly tripled during a period of substantial liberalization of medical and nonmedical cannabis policy in Ontario, Canada, according to a study published in JAMA Network Open.

Over the past 2 decades, Canada has undergone a marked shift in cannabis policy, from allowing limited access to medical cannabis in 2001 to expanding access to anyone with medical authorization in 2014, explained corresponding author Daniel T. Myran MD, MPH, and colleagues. In 2018, Canada became the first country in the world to legalize nonmedical cannabis, with new product commercialization beginning in early 2020.

Researchers pointed out that the major policy changes, combined with robust administrative data for health encounters in Ontario, provided a unique opportunity to investigate associations between cannabis policy liberalization and schizophrenia risk over time. To do so, they tracked the population-attributable risk fraction (PARF) for CUD associated with schizophrenia from 2006 through 2022.

“PARFs estimate the proportion of cases of a condition that could have been prevented if the exposure were eliminated, assuming a causal relationship between exposure and outcome—in our study, the cases of schizophrenia that could have been prevented if cannabis use patterns severe enough to require care in the emergency department or hospital were eliminated,” they wrote.

The population-based cohort study included nearly 13.6 million people aged 14 to 65 years without a history of schizophrenia. Among them, 118,650 had a CUD, of whom 8.9% developed schizophrenia. Meanwhile, 0.6% of people without a CUD developed schizophrenia.

The PARF for CUD associated with schizophrenia increased from 3.7% in 2006 to 10.3% in 2022, according to the study. In the postlegalization period, the PARF ranged from 1.8% among women aged 45 to 65 years to 18.9% among men aged 19 to 24 years.

An analysis of changes over time showed a steady increase, with no acceleration after policy changes, in PARF for CUD associated with schizophrenia. However, PARF increases for CUD associated with psychosis not otherwise specified accelerated after the liberalization of medical cannabis.

“Ongoing research is needed on long-term trends in the incidence of psychotic disorders associated with changes in cannabis policy,” researchers wrote, “particularly the commercialization of the legal cannabis market, which was not well captured in this study.”