Schizophrenia incidence sharply increased following cannabis legalization

1. After cannabis policy legalization, incident cases of schizophrenia linked to cannabis use disorder nearly tripled.

Evidence Rating Level: 2 (Good)

The use of cannabis has been associated with the development of psychosis and schizophrenia. However, the association between cannabis legalization and psychosis is unclear. This retrospective cohort study thus examined how population-attributable risk fraction (PARF) for cannabis use disorder (CUD) associated with schizophrenia changed over time after medical cannabis liberalization and nonmedical cannabis legalization in Canada. Using routinely collected health administrative data of residents aged 14-65 from the province of Ontario, Canada, this study examined annual changes in the PARF of schizophrenia associated with CUD between January 1, 2006, and December 31, 2022. Three policy periods were analyzed: prelegalization (Jan 2006–Nov 2015), cannabis liberalization (Dec 2015–Sep 2018), and nonmedical legalization (Oct 2018–Dec 2022). The primary outcome was diagnosis of schizophrenia and the secondary outcome was diagnosis of psychosis not otherwise specified (NOS). This study included a total of 13,588,681 individuals (mean [SD] age, 39.3 [16.1] years; 6,804,906 males [50.1%]), of whom 118,650 (0.9%) had CUD. In total, 91,106 individuals (0.7%) developed schizophrenia (10,583 of 118,650 [8.9%] with CUD vs 80,523 of 13,470,031 [0.6%] without CUD). The PARF of CUD associated with schizophrenia nearly tripled from 3.7% (95% CI, 2.7%-4.7%) prelegalization to 10.3% (95% CI, 8.9%-11.7%) postlegalization. The overall incidence of schizophrenia per 100,000 individuals remained stable between the prelegalization (53.5) and postlegalization (53.3) periods, while the incidence of psychosis NOS increased by 60.2%, from 33.9 to 54.3. The largest absolute increase in PARF occurred among individuals aged 19 to 24 years, which increased in males from 8.5% (95% CI, 6.7%-10.3%) to 18.9% (95% CI, 16.8%-21.0%) and in females from 4.8% (95% CI, 3.3%-6.3%) to 12.0% (95% CI, 9.5%-14.6%). Overall, this study found that during a period of cannabis policy liberalization, incident cases of schizophrenia linked to CUD nearly tripled. Future research is needed to examine the long-term impact of cannabis policy changes on the incidence of psychotic disorders. 

Click to read the study in JAMA Network Open

Image: PD

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