Two new studies led by Ottawa researcher Dr. Daniel Myran link higher rates of death as well as “concerning increases” in new schizophrenia diagnoses to cannabis use disorder.

One study found that cases of schizophrenia associated with cannabis use disorder almost tripled in Ontario between 2006 and 2022, a period in which people seeking hospital treatment for cannabis use disorder increased by 270 per cent. The second study found that people who had been treated at hospitals for cannabis use disorder were at a significantly higher risk of death compared to the general population.

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Both studies, published this past week in the journal JAMA Network Open, are based on data from Ontario over a period that includes cannabis legalization. Canada, the second country to legalize cannabis (in 2018) and the only country to have built a retail market, has some of the best data for research in the area, Myran said. The growing body of work by Myran and other researchers looking at the impact of cannabis use disorder on health is drawing attention around the world.

“I think broadly there is a lot of interest for people around the world in what the Canadian experience is,” Myran said. “In a way, much of the cannabis policy decisions made internationally are going to come from Canadian data and U.S. data.”

The recent research suggests the Canadian experience includes concerning health impacts.

One focus of the research has been links between cannabis use disorder and schizophrenia as well as psychosis.

The study published this past week found the contribution of cannabis use disorder to new cases of schizophrenia had almost tripled in Ontario over the past 17 years.

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“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,” said Myran, who is a Canada Research Chair in Social Accountability at the University of Ottawa. He is also a scientist and investigator with ICES, the Bruyère Health Research Institute and associate scientist at The Ottawa Hospital.

Cannabis use disorder is defined as a person being unable to stop using cannabis despite it causing serious social and health problems. Many people use cannabis without becoming addicted or having negative health or social impacts.

The number of people in Ontario requiring hospital care for cannabis use disorder has increased by 270 per cent since before the legalization of non-medical cannabis. During the same period — between 2006 and 2022 — new cases of schizophrenia in Ontario in people seeking treatment for cannabis use disorder increased from seven per cent to 16 per cent.

Myran noted that a potential association between cannabis use disorder and psychosis had been a long-standing topic of public health and scientific interest.

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“I think the concern is that daily cannabis use has really increased in the last 15 years and at the same time the potency of cannabis has rapidly risen. Both are what prior literature says are major risk factors for psychosis and schizophrenia,” Myran said.

The study also noted a sharp increase in cases of psychotic disorders “not otherwise specified” (meaning the patient had not been diagnosed with a specific disorder) associated with cannabis use disorder.

The research found a strong association between cannabis use disorder and schizophrenia in young males. An estimated 18.9 per cent of incidents of schizophrenia cases in young males were associated with cannabis use disorder by the end of the study. That suggested some of those cases could have been prevented, Myran said, although he noted the research was “not about causality.”

He also noted that cases of diagnosed schizophrenia had increased in young women, while they had decreased among older adults. He called the rise in rates among young women “cautionary.”

The study by researchers from ICES, The Ottawa Hospital, University of Ottawa’s Department of Family Medicine and Bruyère Health Research Institute included all Ontario residents aged 14 to 65 and eligible for universal health care between 2006 and 2022.

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It looked at three policy time periods: before legalization, after liberalization of medical cannabis and after legalization of non-medical cannabis. A total of 118,650 people had emergency department visits or hospitalizations for cannabis use disorder during that time. Over the study period, nine per cent of people with cannabis use disorder developed schizophrenia, compared to 0.6 per cent of the general population.

The second paper, published on Feb. 6, found a significantly higher risk of death among people who had been treated at hospital for cannabis use disorder.

It included all individuals between the ages of 15 and 105 living in Ontario between 2006 and 2021.

The statistical analysis suggested that people requiring hospital treatment for cannabis use disorder were at an elevated risk of premature death compared to the general population, with the highest increased risk among those from age 25 to 44.

The study has limitations, the authors wrote.

“Although our study cannot established causality, individuals with a hospital-based diagnosis of cannabis use disorder were at elevated risk of death,” they wrote. “Although cannabis use disorder may not be directly responsible, our findings highlight a growing segment of the population who are at elevated risk of death and may benefit from preventative measures.”

The journal published a commentary along with the study on cannabis use disorder and mortality, calling it a “public health crisis.”

https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2829914

https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2829844

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