New schizophrenia cases linked to heavy cannabis use have tripled, particularly in young men
The proportion of new schizophrenia cases linked to cannabis use has nearly tripled in almost two decades, increasing gradually as access to the drug was expanded for medical and later recreational use, according to new population-based research.
Experts say the new study, which looked at more than 13.5 million medical records of all individuals aged 14 to 65 in the Canadian province of Ontario between 2006 and 2022, adds to growing evidence that heavy cannabis use could lead to schizophrenia and psychosis.
“The main thing that we're looking at is whether or not an increasing number of people who have a new diagnosis of schizophrenia have received care for a cannabis use disorder before they got their diagnosis,” Daniel Myran, Canada research chair at the University of Ottawa’s department of family medicine and an investigator at the Bruyère Health Research Institute, told Euronews Health.
“If you look at the study, that increases considerably over time,” said Myran, who was first author of the study.
The proportion of new cases of schizophrenia linked to cannabis use in the province increased from 3.7 per cent before it was legalised to 10.3 per cent after it was legalised for non-medical use in Canada in 2018, the researchers said.
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The study was published on Tuesday in the journal JAMA Network Open.
While the incidence of schizophrenia “was stable over time” without a population-wide increase, the incidence of psychosis that was not otherwise specified increased compared to before marijuana was legalised.
In certain age groups, particularly among young men, the incidence of schizophrenia did appear to be rising in Ontario.
Nearly 19 per cent of new schizophrenia cases in young men aged 19 to 24 were linked to cannabis use disorder at the end of the study, researchers said.
“I think the study is a reminder that, no, this is not necessarily safe or fine for everyone and that in particularly young people whose brains continue to develop, there is a very strong association between cannabis use and psychosis and schizophrenia,” Myran said.
‘Mounting evidence’ heavy cannabis use linked to psychosis
Previous research has also pointed to a link between heavy cannabis use and schizophrenia.
A study published in JAMA Psychiatry in 2021, which included more than 7 million people born before 2001 in Denmark, found that the proportion of schizophrenia cases linked to cannabis use disorder increased three to fourfold over the last two decades, which the researchers said was expected due to “increases in the use and potency of cannabis”.
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A 2019 study of 11 sites across Europe also found that daily cannabis use, especially when it had a high potency, was linked to developing psychosis.
Marta Di Forti, a professor of drugs, genes, and psychosis at King’s College London who led the 2019 study, said the new Canadian research was “exceptional” and robust due to its sample size and longitudinal nature.
Di Forti, who was not involved in the new research, has seen patients who are heavy cannabis users develop psychosis.
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“Their psychosis manifests predominately with a sort of paranoia, with the certainty and the fear that the environment around the individual is hostile and threatening to them,” she told Euronews Health.
“You can imagine how this is alienating the person from the outside world and is also making the person constantly fearful, which sometimes can manifest with completely withdrawing from interaction or becoming quite aggressive,” she added.
‘Huge cost’
The study had several limitations, including researchers not having data on genetics, adverse childhood experiences, and other potential factors that could contribute to schizophrenia.
They also only looked at people who were treated for cannabis use disorder in the hospital or emergency department, so the team could also be underestimating the link.
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In a commentary published alongside the study, Jodi Gilman, an associate professor in psychiatry at Harvard Medical School, who was not associated with the study, wrote that its findings add “further support for mounting evidence on the association between cannabis use and increased risk for psychosis”.
The study “shows that this association is most robust among young adults who are developmentally vulnerable to both the neurologic effects of cannabis and developing psychosis,” she said.
Di Forti qualified that the research should not minimise the potential of cannabis for therapeutic use under certain conditions but said that if countries legalise cannabis, they need to provide care for people who develop psychosis or cannabis dependence.
For her, the study’s overall findings show this is “a huge cost to mental health services, huge cost to family and society, [and a] huge cost to young people”.
