Cannabis legalization linked to rise in related schizophrenia cases in Canada
Key takeaways:
- New schizophrenia cases associated with cannabis use disorder nearly tripled in Ontario after cannabis legalization.
- This association was greatest among men aged 19 to 24 years.
The incidence of cannabis use disorder associated with schizophrenia almost tripled among Canadian teenagers and adults after cannabis legalization, according to a study published in JAMA Network Open.
“Our study highlights the growing public health challenge posed by the combination of increasingly high-potency cannabis and rising regular cannabis use,” Daniel T. Myran, MD, MPH, CCFP, FRCPC, Canada Research Chair in Social Accountability at University of Ottawa, adjunct scientist at the Institute for Clinical Evaluative Sciences, investigator at Bruyère Health Research Institute and associate scientist at The Ottawa Hospital, said in a related press release.
Although studies have shown that cannabis use is linked to subsequent development or early onset of schizophrenia, research has been limited concerning the association between cannabis legalization and schizophrenia.
Myran and colleagues conducted a population-based retrospective cohort study of 13.5 million Ontario residents aged 14 to 65 years (49.9% female; mean age, 39.3 years; standard deviation [SD] = 16.1 years) who were eligible for the Ontario Health Insurance Plan without a diagnosis of schizophrenia. They aimed to examine changes over time in population-attributable risk fraction (PARF) for cannabis use disorder (CUD) associated with schizophrenia or psychotic disorder not otherwise specified (NOS) following medical cannabis liberalization and nonmedical cannabis legalization.
The researchers followed the participants for 17 years over three policy periods:
- Prelegalization: January 2006 to November 2015;
- Liberalization of medical and nonmedical cannabis: December 2015 to September 2018; and
- Legalization of recreational cannabis: October 2018 to December 2022.
Of the total population, 118,650 (0.9%) had CUD, as indicated by one or more ED visit or hospitalization due to cannabis use. The researchers found that individuals with CUD were more likely to be male, younger and live in low-income neighborhoods.
Based on a medical record-validated algorithm, the researchers further found that 91,106 (0.7%) individuals developed schizophrenia throughout the study period; 10,583 of 118,650 (8.9%) with CUD and 80,523 of 13,470,031 (0.6%) without CUD.
Between 2006 and 2022, the researchers found that the annual age- and sex-standardized incidence of schizophrenia decreased by 27.2%, whereas CUD increased by 497.4% and psychosis NOS increased by 83.7%.
Notably, incident cases of schizophrenia rose from 7% in the prelegalization period to 16.7% during the legalization period among patients who were treated at a hospital for CUD in the prior 3 years.
The researchers also found that the adjusted PARF for CUD associated with schizophrenia nearly tripled from 3.68% (95% CI, 2.72%-4.65%) prelegalization to 10.27% (95% CI, 8.86%-11.68%) during the legalization period.
Further, the researchers found that PARF differed substantially by age and sex during the legalization period, with the highest rates among men aged 19 to 24 years, at 18.88% (95% CI, 16.79%- 20.97%), and the lowest among women aged 45 to 65 years, at 1.81% (95% CI, 1.06%-2.56%).
Additionally, when analyzing an interrupted time series model, the researchers noted that PARF for CUD associated with schizophrenia increased steadily over the study period, at 0.1% per quarter — with no accelerations due to policy changes — whereas increases in PARF for CUD associated with psychosis NOS accelerated after medical cannabis liberalization, from 0.1% to 0.3%.
“The tripling of schizophrenia cases associated with a CUD over the past 17 years and rising cases of psychosis underscores the urgent need for targeted prevention strategies, particularly for younger populations who appear to be at the greatest risk,” Myran said in the release.
In a related editorial, Jodi M. Gilman, PhD, director of neuroscience at Massachusetts General Hospital Center for Addiction Medicine and associate professor of psychiatry at Harvard Medical School, wrote that the study’s findings further support existing evidence on the link between risk for psychosis associated with cannabis use, especially among young adults “who are developmentally vulnerable to both the neurologic effects of cannabis and developing psychosis.”
She also noted that there are substantial obstacles to effectively studying the mental health consequences of cannabis policy changes through large-scale electronic medical record studies. These include determining the most applicable time point to analyze the association between legalization and outcomes, reliably identifying the timing of cannabis initiation and psychosis onset, and calculating risk for psychosis without details on THC potency.
Gilman also noted several limitations to the study, including that patients who did not seek care for CUD or who received outpatient care for CUD would be misclassified as not having CUD.
“Epidemiologic studies should include long time intervals to account for processes of legalization and the emergence of mental illness, sufficient numbers to identify subpopulations at risk, such as young adults, and information concerning not only the quantity and frequency but also the potency of cannabis used,” Gilman wrote. “Without these critical factors, this research will be skewed toward nonsignificant findings, potentially obscuring important associations between cannabis policy and mental health outcomes.”
References:
- Contribution of cannabis use disorder to new cases of schizophrenia has almost tripled over the past 17 years. https://www.ices.on.ca/news-releases/contribution-of-cannabis-use-disorder-to-new-cases-of-schizophrenia-has-almost-tripled-over-the-past-17-years/. Published Feb. 4, 2025. Accessed Feb. 5, 2025.
- Gilman JM. JAMA Netw Open. 2025;doi:10.1001/jamanetworkopen.2024.57876.
Perspective
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Although the study design was observational and therefore causation cannot be inferred, this 17-year study following more than 13 million Canadians suggests that at the population level, CUD may be associated with an elevated risk for schizophrenia. These data can help public health policymakers make evidence-based decisions regarding the availability, legality and accessibility of medical and recreational cannabis.
More large, longitudinal investigations are needed to clarify the relationship between CUD and psychotic disorders, particularly surrounding the individual-level clinical and biopsychosocial factors that may place some at higher risk for psychotic disorders than others (eg, age, trauma, polysubstance use, familial history).
Future studies may benefit from including a wide range of psychotic disorders as outcomes given the study’s findings on the accelerated rates of psychosis NOS, as well as from evaluating how the specific properties of cannabis products (eg, THC content, formulations, routes of administration) and the patterns of cannabis use at the individual level, relate to these elevated risks.
School of Professional and Applied Psychology
Director of psychosocial research
Medical Marijuana Research Program
Philadelphia College of Osteopathic Medicine
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