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Daniel Myran is a family physician, an assistant professor at the University of Ottawa’s Department of Family Medicine and an investigator with the Bruyère Research Institute, the Ottawa Hospital Research Institute and ICES. His research focuses on the inequities of substance use and the impacts of drug policy in Canada.

Cannabis use is often portrayed as relatively harmless compared to the use of other substances, but a growing body of research supports the notion that cannabis use may have a serious impact on people’s mental health – particularly for young men.

Concerns about cannabis use and mental health are not new. There are decades-long debates about whether cannabis use increases the risk for psychosis and schizophrenia, and there is increasing interest in the link between cannabis use and anxiety and depression. These debates boil down to a chicken-and-egg problem: does cannabis use increase the risk of mental illness, or are people with mental illness using cannabis to try and self-manage distressing symptoms? Another possibility is that shared genetic and social risk factors predispose some individuals to both cannabis use and mental health conditions.

My research team recently conducted two large Canadian studies examining the relationship between cannabis use and mental health. Our findings are that cannabis use may be implicated in the development of certain types of mental illness.

In the first study, we found that 26 per cent of individuals visiting the emergency department (ED) for an episode of cannabis-induced psychosis developed schizophrenia within three years – a rate 242 times higher than the general population.

In the second study, we found that 27 per cent of individuals who visited the ED for cannabis use were diagnosed with an anxiety disorder within three years, compared to six per cent of the general population – a rate almost four times higher after taking into account differences in demographics and previous care for mental health and substance use. ED visits for cannabis use were also associated with an increased risk for severe or worsening anxiety in people who already had an anxiety disorder.

Similar research from Denmark has found that individuals with treatment for cannabis-use disorder were at nearly double the risk of depression and three times the risk of bipolar disorder.

Youth and younger adults (under 25 years of age) – particularly young men – appear to be at the most elevated risk of adverse mental health outcomes. In men aged 14 to 24, 40 per cent developed schizophrenia within three years of an episode of cannabis-induced psychosis – double the risk for women of the same age. Similarly, we found that the association between ED visits for cannabis use and being diagnosed with an anxiety disorder was strongest in younger adults and men. The reasons for these differences are not fully understood but are likely related to the susceptibility of developing brains to the negative effects of cannabis.

These studies are by no means conclusive, and in the chicken-and-egg debate, both sides are probably capturing an element of truth. However, the warning signal that cannabis use may cause mental health problems should not be taken lightly, as cannabis use in Canada is rapidly growing. Data from surveys support that the number of Canadians who use cannabis has doubled over the past 15 years. In 2023, 42.8 per cent of Canadians aged 16 to 19 and 47.6 per cent of Canadians aged 20 to 24 reported using cannabis in the past year, with nine per cent of 16- to 19-year-olds and 11 per cent of 20- to 24-year-olds reporting daily or almost-daily cannabis use.

Not only is cannabis use growing, but the risk profile of available products is increasing as well. Higher levels of tetrahydrocannabinol (THC), the main psychoactive component of cannabis, have been linked with increased risk of addiction and mental health concerns. Today, most legally sold cannabis in Canada contains over 20 per cent THC, roughly 10 times higher than it was between the 1960 and 1980s (around two per cent) and double what it was a decade ago.

Given mounting evidence of potential links between cannabis use and harms, and our current youth mental health crisis, precautionary approaches to reduce cannabis use in youth seem warranted. Increasing education about the potential risks of cannabis use is a start, especially for young people who appear more vulnerable to adverse mental health effects. An advantage of having recently legalized non-medical cannabis is that the government heavily influences the market. It’s arguably time to tighten policies that can reduce youth cannabis use and the use of higher-potency products.

At the end of the day, cannabis is not a harmless substance, and our societal conversations and approach to policy need to more clearly reflect that reality.

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