Changes in Incident Schizophrenia Diagnoses Associated With Cannabis Use Disorder In Canada Post-Legalization

"Over the past 17 years, the proportion of new cases of schizophrenia in Ontario, Canada, associated with diagnosed CUD [Cannabis Use Disorder] has increased from 1.6% in 2006 to 9.6% in 2022. The percentage of new cases of schizophrenia associated with CUD increased gradually over time and did not accelerate after the liberalization of medical cannabis or the legalization of nonmedical cannabis. Increases in the percentage of new cases of psychosis NOS [Not Otherwise Specified] associated with CUD accelerated after medical cannabis legalization. The risk of developing schizophrenia associated with CUD was fairly stable over time, and the increases in the PARF [Population-Attributable Risk Fraction] were associated primarily with an increasing prevalence of CUD in the population. There were large variations in the PARF by age and sex, with a much larger proportion of cases of schizophrenia associated with CUDs among youths, particularly among males.

"The increasing PARF for CUD associated with schizophrenia are consistent with population-representative surveys on cannabis use in Ontario. Between 2006 and 2022, the percentage of Ontario residents aged 18 years or older who reported past-year cannabis use and patterns placing them at moderate or high risk of experiencing social problems or cannabis dependence increased by 145.5% (from 13.4% in 2006 to 32.9% in 2022) and 223.3% (from 6.0% in 2006 to 19.4% in 2022), respectively.28 The THC content of cannabis in North America has more than doubled over our study time frame, with more than 70% of legal dried cannabis sold in Ontario currently exceeding 20% THC.29 Consistent with increasing cannabis use and potency, ED visits in Ontario for cannabis-induced psychosis and CUD have increased over time.24,29 Our results, and those of others, highlight that the association between CUD and schizophrenia may be particularly elevated among younger males, in whom an estimated 18.9% of incident schizophrenia cases were associated with CUD by the end of the study.30

"Specific to cannabis policy, we observed no change in the prelegalization trend of increasing PARF over time for our primary outcome after medical cannabis liberalization and nonmedical cannabis legalization with restrictions. The PARF of CUD associated with our secondary outcome (psychosis NOS) did accelerate after medical cannabis liberalization in 2015. These findings should be interpreted with 2 caveats. First, attribution of the associations of changes in cannabis policy from changing social norms and behaviors in the lead-up to changes is challenging. Although evidence from the US and Canada has found large increases over time in CUDs, the association of legalization with these trends has been modest, with increases also occurring in regions without legal cannabis.24,31 Prior work has supported that cannabis use in Canada began increasing prelegalization when the government announced it intended to legalize cannabis and there were increasing societal discussions and acceptance of cannabis.32 Second, our study does not fully evaluate cannabis legalization, as the market was heavily restricted before 2020, leaving less than 2 years of higher retail and high-potency product exposure for individuals to develop new CUDs, develop schizophrenia, and receive a formal diagnosis. The final 2 years of our study also overlapped with the COVID-19 pandemic, which resulted in changes in health service use. Evidence suggests that greater exposure to cannabis retail stores and higher-potency cannabis products is associated with increased cannabis use and harms and ongoing monitoring is indicated.8,33,34"