Scientists have suggested that Canada’s legalisation of cannabis may be linked to an escalation in schizophrenia cases, according to their research findings.
Over a 16-year period – spanning before and after legalisation of the drug for medical and recreational use – the number of new cases of schizophrenia associated with cannabis use disorder (CUD) in Ontario nearly tripled after legalisation, the investigators found.
“There has been a lot of research on the association between cannabis use and schizophrenia and one of the main concerns about legalisation is whether it might result in increases in cases,” study investigator Daniel Myran, MD, MPH, with Ottawa Hospital Research Institute, told Medscape Medical News.
“Our findings show 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 was published online in JAMA Network Open.
Growing public health challenge
The researchers evaluated the electronic medical records of more than 13.5m residents of Ontario (mean age, 39 years) without a history of schizophrenia and considered three policy time periods between 2006 and 2022 – before legalisation (January 2006 to November 2015), after legalisation of medical cannabis (December 2015 to September 2018), and after legalisation of non-medical 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 legalisation of cannabis, the number of people in Ontario who required hospital care for CUD jumped by 270%, from about 1.3 in every 1 000 people before legalisation to 4.6 after legalisation, 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 legalisation to 10.3% (95% CI, 8.9%-11.7%) after legalisation, with young men aged 19-24 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.
He added that “part of the challenge … is that with all the discussion around medical cannabis, people may think that ‘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 there is no risk, particularly in younger people.
“Alcohol and tobacco are legal but that doesn’t mean those products are safe or without consequences,” he noted.
The investigators said 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, 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 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 legalisation … 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.
“Epidemiologic studies should include long time intervals to account for processes of legalisation 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 non-significant findings, potentially obscuring important associations between cannabis policy and mental health outcomes,” Gilman said.
Study details
Changes in Incident Schizophrenia Diagnoses Associated With Cannabis Use Disorder After Cannabis Legalisation
Daniel Myran, Michael Pugliese, Lyndsay Harrison, et al.
Published in JAMA Network Open on 4 February 2025
Abstract
Importance
Despite public health concerns that cannabis legalisation may increase the number of cases of schizophrenia caused by cannabis, there is limited evidence on this topic.
Objective
To examine changes in the population-attributable risk fraction (PARF) for cannabis use disorder (CUD) associated with schizophrenia after liberalisation of medical cannabis and legalisation of non-medical cannabis in Canada.
Design, Setting, and Participants
This population-based cohort study was conducted in Ontario, Canada, from January 1, 2006, to December 31, 2022, among 13 588 681 people aged 14 to 65 years without a history of schizophrenia.
Exposures
Diagnosis of CUD in the emergency department or hospital setting (International Statistical Classification of Diseases and Related Health Problems, Tenth Revision, Canada [ICD-10-CA] codes F12x and T40.7).
Main Outcome and Measures
Changes in the PARF for CUD associated with schizophrenia (ICD-10-CA codes F20x and F25x and Diagnostic and Statistical Manual of Mental Disorders [Fourth Edition] [DSM-IV] code 295x) over 3 policy periods: prelegalisation (January 2006 to November 2015), liberalisation of medical and non-medical cannabis (December 2015 to September 2018), and legalisation of non-medical cannabis (October 2018 to December 2022). A secondary outcome was diagnosis of psychosis not otherwise specified (NOS) (ICD-10-CA code F29x and DSM-IV code 298x). Segmented linear regression was used to examine changes after the liberalisation of medical cannabis in 2015 and the legalisation of non-medical cannabis in 2018.
Results
The study included 13 588 681 individuals (mean [SD] age, 39.3 [16.1] years; 6 804 906 males [50.1%]), of whom 118 650 (0.9%) had CUD. A total of 91 106 individuals (0.7%) developed schizophrenia (80 523 of 13 470 031 [0.6%] in the general population without CUD vs 10 583 of 118 650 [8.9%] with CUD). The PARF for CUD associated with schizophrenia almost tripled from 3.7% (95% CI, 2.7%-4.7%) during the prelegalisation period to 10.3% (95% CI, 8.9%-11.7%) during the legalisation period. The PARF in the post-legalisation period ranged from 18.9% (95% CI, 16.8%-21.0%) among males aged 19 to 24 years to 1.8% (95% CI, 1.1%-2.6%) among females aged 45 to 65 years. The annual incidence of schizophrenia was stable over time, while the incidence of psychosis NOS increased from 30.0 to 55.1 per 100 000 individuals (83.7%) in the postlegalisation period relative to the prelegalisation period. The PARF for CUD associated with schizophrenia increased steadily over the study with no accelerations after cannabis policy changes, while increases in the PARF for CUD associated with psychosis NOS accelerated after medical cannabis liberalisation.
Conclusions and Relevance
In this cohort study of individuals aged 14 to 65 years in Ontario, Canada, the proportion of incident cases of schizophrenia associated with CUD almost tripled during a period of substantial liberalization of cannabis policy. Ongoing research is indicated to understand the long-term associations of cannabis policy with the prevalence of psychotic disorders.
JAMA Network article – Has legalised cannabis led to a surge in schizophrenia cases? (Open access)
Medscape article – Has legalised cannabis led to a surge in schizophrenia cases? (Open access)
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