A new study from Canadian researchers has found that the proportion of new schizophrenia cases associated with cannabis use disorder (CUD) has more than doubled in Ontario since the legalization of cannabis.
Before legalization, 4% of new schizophrenia cases were linked to CUD, but that number has now risen to 10%. Among young men aged 14 to 24, the percentage is even higher, reaching 18%.
The study, published in JAMA Network Open, was conducted by researchers from ICES, The Ottawa Hospital, the University of Ottawa, and the Bruyère Health Research Institute.
Using health data from more than 13.5 million Ontario residents between 2006 and 2022, the researchers examined whether changes in cannabis policy—such as the liberalization of medical cannabis in 2015 and the full legalization of recreational cannabis in 2018—were linked to changes in schizophrenia cases associated with cannabis use disorder.
Key Findings
The study revealed several concerning trends:
- The number of people in Ontario needing hospital care for CUD has increased by 270% since cannabis was legalized, rising from 1.3 in 1,000 people to 4.6 in 1,000 people.
- The percentage of schizophrenia cases linked to CUD increased from 7% to 16% over the same period.
- Researchers estimated that 10% of new schizophrenia cases could have been prevented if individuals with severe cannabis use disorder had stopped using cannabis.
- Among young men aged 14 to 24, the percentage of schizophrenia cases linked to CUD was even higher at 18%.
Why These Findings Matter
Schizophrenia is a serious mental illness that affects how a person thinks, feels, and behaves. It can cause hallucinations, delusions, and cognitive difficulties, making daily life challenging.
While scientists continue to debate whether heavy cannabis use directly causes schizophrenia, there is strong evidence that it worsens symptoms and leads to poorer long-term outcomes for those already at risk.
Dr. Daniel Myran, the study’s lead researcher and a Canada Research Chair at the University of Ottawa, highlighted the growing concerns about high-potency cannabis and increasing regular use.
“Our study shows a troubling rise in schizophrenia cases linked to cannabis use disorder. This underscores the need for targeted prevention strategies, particularly for younger people, who appear to be at the greatest risk,” he said.
What Needs to Happen Next?
The sharp rise in hospitalizations and schizophrenia cases associated with cannabis use disorder suggests that public health officials need to take action. Prevention efforts should focus on educating young people about the risks of heavy cannabis use, particularly as cannabis products become more potent.
Mental health professionals should also be aware of the increasing number of patients experiencing psychosis linked to cannabis use.
While legalization has made cannabis more accessible, it has also brought new challenges. This study highlights the need for continued research and policies that balance access to cannabis with public health concerns.
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The research findings can be found in JAMA Network Open.
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