Ottawa, ON, February 4, 2025 – The proportion of new cases of schizophrenia associated with a cannabis use disorder has risen from 4% pre-legalization to 10% after cannabis legalization in Ontario, according to new research.
A new study from researchers at ICES, The Ottawa Hospital, University of Ottawa's Department of Family Medicine, and Bruyère Health Research Institute and published in the journal JAMA Network Open used data capturing the healthcare visits of everyone living in Ontario, Canada to track whether the liberalization of medical cannabis in 2015 and legalization of non-medical cannabis in Canada in 2018 were linked to changes in how much cannabis use disorders (CUDs) were associated with new schizophrenia cases.
"Regular cannabis use is strongly associated with an increased risk of schizophrenia, and one of the main areas of uncertainty surrounding cannabis legalization is whether there would be changes in the number of new cases of schizophrenia," says Dr. Daniel Myran, a Canada Research Chair in Social Accountability at the University of Ottawa, ICES Adjunct Scientist, Investigator at the Bruyère Health Research Institute, and Associate Scientist at The Ottawa Hospital.
"We found that there have been concerning increases over time in the percentage of people with a new schizophrenia diagnosis who had received care for a cannabis use disorder before their diagnosis."
The study included all Ontario residents aged 14 to 65 years and eligible for universal health care (over 13.5 million individuals) and considered three policy time periods between 2006 and 2022: before legalization, after liberalization of medical cannabis, and after legalization of non-medical cannabis.
A total of 118,650 individuals (0.9% of the population of Ontario) had an emergency department visit or hospitalization for a CUD. Over the study period, 10,583 (9%) of individuals with CUD developed schizophrenia compared to 80,523 (0.6%) of individuals without CUD.
Key Findings
The number of individuals in Ontario who require hospital care for a CUD has increased by 270% (1.3 in 1,000 people to 4.6 in 1,000 people) since before legalization to after the legalization of non-medical cannabis.
Over the same period, the percentage of all new cases of schizophrenia in Ontario who had received hospital care for a CUD before their schizophrenia diagnosis increased from 7% to 16%.
After adjusting for differences between individuals with and without hospital care for a CUD the authors estimated that during the legalization period, 10% of new cases of schizophrenia could have been prevented if people with cannabis use severe enough to require care in the emergency department or hospital had not stopped using cannabis. In men aged 14 to 24 that percentage rose to 18%.
"Our study highlights the growing public health challenge posed by the combination of increasingly high-potency cannabis and rising regular cannabis use," says Dr. Myran.
The authors highlight that the study does not settle ongoing debates about whether or not heavy cannabis use can cause schizophrenia. Nevertheless, the authors note that heavy cannabis use does worsen symptoms and the prognosis for those living with schizophrenia and caution about the trends observed in the study.
"The tripling of schizophrenia cases associated with a cannabis use disorder 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," says Dr. Myran.
The article "Changes after cannabis legalization in incident schizophrenia diagnoses associated with cannabis use disorder in Canada," was published in JAMA Network Open.
