A new analysis of birth cohorts in the Canadian province of Ontario has found a striking rise in the incidence of psychotic disorders among young people, a finding that its authors said could reflect teens’ increasing use of substances like cannabis, stimulants and hallucinogens.
The study, published on Monday in The Canadian Medical Association Journal, found that the rate of new diagnoses of psychotic disorders among people ages 14 to 20 increased by 60 percent between 1997 and 2023, while new diagnoses at older ages plateaued or declined.
Compared with people born in the late 1970s, those born in the early 2000s were about twice as likely to have been diagnosed with a psychotic disorder by age 20. The researchers included 12 million people born in Ontario between 1960 and 2009, of which 0.9 percent were diagnosed with a psychotic disorder during the study period.
The study was epidemiological and did not try to identify a cause for the rising prevalence. There are a number of possible explanations, among them older paternal age, the stress of migration, neonatal health problems and early intervention programs that now regularly identify the disorders at younger ages, the authors note.
But Dr. Daniel Myran, one of the study’s authors, said he undertook the study, in part, to follow up on concerns that the legalization of cannabis might increase population-level rates of schizophrenia and other psychotic disorders.
“I was expecting to see some increases in these younger folks, but I was quite surprised by the scale,” said Dr. Myran, a family physician and research chair at North York General Hospital.
He said the results suggested a need for more research into the impact of expanding cannabis use by young people.
In 2024, 41 percent of Canadians ages 16 to 19 reported using cannabis at least once in the last year, up from 36 percent in 2018, the year the drug was legalized, according to the Canadian Cannabis Survey.
“I think that there is a group of individuals who develop psychosis because of substance use, who in its absence would not have,” Dr. Myran said. “The interesting question is, how much of psychosis is caused by a modifiable substance use? Are these edge scenarios, where it’s pretty rare? Or is it actually quite frequent?”
Scientists in the field have long debated whether cannabis use may cause psychotic disorders or simply accompany them. Drug use often accompanies other stressors that are also known risk factors for psychosis, and people with psychotic disorders may use cannabis to manage their symptoms.
Increases in incidence of psychosis in young people have been reported in northern Europe, but not in the United States, and in interviews, several experts said it would be important to replicate the findings of the Canadian study before drawing firm conclusions.
“No doubt this analysis is an outlier in showing a sharp rise in new cases,” said Dr. Dost Ongur, chief of the division of psychotic disorders at McLean Hospital, a psychiatric hospital outside of Boston, who was not involved in the study. Though other studies have found rising rates of psychosis, the shifts were “much milder,” and more evidence was necessary to prove that cannabis was causing the increase, he said.
Dr. Deepak D’Souza, a professor of psychiatry at Yale who was not involved in the study, said the new findings “suggest that psychosis may be occurring more frequently, being detected earlier by the health system, or both.” Any evidence suggesting an increase in psychosis, he said, “is both alarming and deserving of careful scrutiny.”
One reason U.S. researchers may not have detected a similar trend is that the United States lacks a universal health system like Canada’s, which captures rich data on medical care for the entire population over time. That allows researchers to compare prevalence among birth cohorts, Dr. Myran said.
The Ontario study looked at schizophrenia, a severe disorder characterized by delusions and altered thinking, and at related diagnoses. The largest increase was driven by “psychosis not otherwise specified,” a category typically used when psychosis is evident but there is not a definitive diagnosis of schizophrenia.
Psychotic disorders are known to be heavily genetic and their prevalence is relatively stable, affecting about 1 percent of the population. But differences became apparent when the researchers separated out young people from the overall population, Dr. Myran said. As the rate of new diagnoses among young people rose, rates among older people were declining, masking the effect, he said.
As evidence linking cannabis use and psychotic disorders has emerged, skeptics within psychiatry have countered that if the link is true the prevalence of psychotic disorders should be rising, said Dr. Kevin Gray, a specialist in bio-behavioral medicine at Medical University of South Carolina Health, who was not involved in the study. “This is such an elegant design, and it may answer that question,” he said, referring to the new Canadian study.
But, he cautioned, the finding “begs for replication across other jurisdictions.”
“We don’t have the smoking gun to say this is the mechanism,” he said.
Dr. Myran said that while the mechanisms were not yet fully understood, it might be the case that regular use of substances during adolescence drives “rewiring or structural changes” in the brain.
“You go to the idea that neurons that fire together wire together,” he said. “Your brain is quite plastic, and if we have substance use that’s causing hallucinations, if you have ongoing activation and firing and that kind of stuff, it can change your brain.”
Ellen Barry is a reporter covering mental health for The Times.
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