An Ontario study found that cases of schizophrenia associated with cannabis use disorder have doubled since cannabis was legalized.
Hospital visits for cannabis-related problems increased by 270%, with young men being the most vulnerable.
Legalization of marijuana and increase in cases of schizophrenia
New cases of schizophrenia linked to cannabis use disorder (CUD) have doubled in Ontario, from 4% before legalization to 10% after, according to new research.
A study published today (February 4) in JAMA Network Open by researchers at ICES, The Ottawa Hospital, the University of Ottawa Department of Family Medicine, and the Bruyère Health Research Institute analyzed health care data from all Ontario residents. The researchers examined whether the liberalization of medical cannabis in 2015 and the legalization of non-medical cannabis in 2018 affected the association between cannabis use disorder (CUD) and new diagnoses of schizophrenia.
Uncertainty and risks of cannabis use
“Regular cannabis use is strongly associated with an increased risk of schizophrenia, and one of the main areas of uncertainty around cannabis legalization is whether there will be a change in the number of new cases of schizophrenia,” said Dr. Daniel Myran, Canada Research Professor in Social Responsibility at the University of Ottawa, Visiting Scientist at ICES, Bruyère Institute for Health Research and Associate Scientist at The Ottawa Hospital.
“We found a worrying increase over time in the proportion of people newly diagnosed with schizophrenia who had received treatment for a cannabis use disorder before being diagnosed.”
The study included all Ontario residents aged 14 to 65 and eligible for universal health care (more than 13.5 million people) and examined three policy periods from 2006 to 2022: before legalization, after liberalization of medical cannabis, and after legalization of non-medical cannabis.
A total of 118,650 people (0.9% of the Ontario population) visited the emergency department or were hospitalized for TCC. During the study period, 10,583 (9%) people with TCC developed schizophrenia compared with 80,523 (0.6%) people without TCC.
Main findings
- The number of people in Ontario requiring hospital care for TCC increased by 270% (1.3 per 1,000 people to 4.6 per 1,000 people) from before legalization to after legalization of non-medical cannabis.
- During the same period, the proportion of new schizophrenia cases in Ontario who received inpatient CBT care before being diagnosed with schizophrenia increased from 7% to 16%.
- After adjusting for differences between individuals with and without inpatient CBT care, the authors estimated that during the legalization period, 10% of new cases of schizophrenia could be prevented if cannabis users with an illness severe enough to require emergency department or hospital care did not stop using cannabis. For males aged 14 to 24, that rate rises to 18%.
High-potency cannabis and public health challenges
“Our study highlights the growing public health challenge posed by the combination of high-potency cannabis and increasing regular cannabis use,” said Dr. Myran.
The authors emphasize that this study does not resolve current debates about whether heavy cannabis use can cause schizophrenia. However, the authors note that heavy cannabis use worsens symptoms and outcomes for people with schizophrenia and are cautious about the trends observed in the study.
Call for a preventive strategy
“The tripling of schizophrenia cases associated with cannabis use disorders over the past 17 years and the increase in psychotic cases underline the urgent need for targeted prevention strategies, particularly for the younger population, who are at higher risk,” said Dr. Myran.
Reference: “Changes in the diagnosis of schizophrenia associated with cannabis use disorder”
