Has Legal Cannabis Led to Surge in Schizophrenia?

Canada’s legalization of cannabis has been linked to an increase in schizophrenia cases, new research shows.

Over a 16-year time period — spanning before and after legalization of cannabis for medical and recreational use — the number of new cases of schizophrenia associated with cannabis use disorder (CUD) in Ontario nearly tripled after legalization, investigators found.

“There has been a lot of research on the association between cannabis use and schizophrenia and one of the main concerns about cannabis legalization is whether it might result in increases in cases of schizophrenia,” study investigator Daniel Myran, MD, MPH, with Ottawa Hospital Research Institute, Ottawa, Ontario, Canada, told Medscape Medical News.

“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,” Myran added in a news release.

The study was published online on February 4 in JAMA Network Open.

A Growing Public Health Challenge

The researchers evaluated the electronic medical records of more than 13.5 million residents of Ontario (mean age, 39 years) without a history of schizophrenia and considered three policy time periods between 2006 and 2022 — before legalization (January 2006 to November 2015), after legalization of medical cannabis (December 2015 to September 2018), and after legalization of nonmedical cannabis (October 2018 to December 2022). 

In total, 118,650 individuals (0.9%) had an emergency department visit or hospital stay for CUD. During the study period, 10,583 (9.0%) of individuals with CUD developed schizophrenia compared with 80,523 (0.6%) of individuals without CUD.

After legalization of cannabis, the number of people in Ontario who required hospital care for CUD jumped by 270%, from about 1.3 in every 1000 people before legalization to 4.6 after legalization, results showed.

In addition, the proportion of new cases of schizophrenia associated with CUD increased from 3.7% (95% CI, 2.7%-4.7%) before legalization to 10.3% (95% CI, 8.9%-11.7%) after legalization, with young men aged 19-24 years most likely to be affected.

“Our study highlights the growing public health challenge posed by the combination of increasingly high-potency cannabis and rising regular cannabis use,” Myran said in the news release.

Myran added that “part of the challenge with cannabis is that with all the discussion around medical cannabis, people may think if this is a medicine it can’t possibly be hurting me. I think the conversation has to be — just because cannabis is now legal and the social norms are changing, doesn’t mean that there is not at risk, particularly in younger people.”

“Alcohol and tobacco are legal and that doesn’t not mean that those products are safe or without consequences,” he noted.

The investigators pointed out that the study does not settle ongoing debate about whether or not heavy cannabis use can cause schizophrenia.

However, Myran said, “what is clear from the scientific literature is that people who are going to develop schizophrenia if they use cannabis and they use it regularly, they will develop it earlier in life and their symptoms will be worse.”

A Natural Experiment

The author of an invited commentary said this study “adds further support for mounting evidence on the association between cannabis use and increased risk for psychosis, and it shows that this association is most robust among young adults who are developmentally vulnerable to both the neurologic effects of cannabis and developing psychosis.”

“As legalization of cannabis becomes more widespread, along with a rapidly expanding commercial cannabis market, a natural experiment of population exposure to commercial cannabis markets is occurring,” wrote Jodi Gilman, PhD, with the Department of Psychiatry, Massachusetts General Hospital and Harvard Medical School, Boston.

“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.

“Without these critical factors, this research will be skewed toward nonsignificant findings, potentially obscuring important associations between cannabis policy and mental health outcomes,” Gilman said.

The study was supported by the Canadian Institutes of Health Research and by ICES, which is funded by an annual grant from the Ontario Ministry of Health and the Ministry of Long-Term Care. Myran and Gilman had no relevant disclosures.