A new and comprehensive study has bolstered the case that there is a strong association between cannabis use and schizophrenia, especially among young men.
The study is novel for its size of nearly seven million people studied. It found that “one-fifth of cases of schizophrenia among young males might be prevented by averting cannabis use disorder” — which refers to a problematic pattern of cannabis use that leads to clinically significant impairment or distress.
The study, conducted in Denmark, does not itself establish causality. However, it adds to a growing body of evidence that there is a strong association between cannabis use and schizophrenia.
“There are a number of other studies, including prospective cohort studies that follow people over time, that yield similar findings,” said David Hammond, a professor at the University of Waterloo School of Public Health Sciences. “The association between regular cannabis use and the development of schizophrenia is one of the health effects with robust evidence.”
“In terms of policy implications, [schizophrenia risk] is among the most persuasive arguments for trying to delay early cannabis use among youth,” Hammond added.
Yet, a study published by researchers at the University of Waterloo suggests that most daily cannabis users are ignorant of the risks.
The younger the user, ‘the higher the risk’
The aim of the Danish study was to investigate the associations between cannabis use disorder and schizophrenia by users’ age and sex.
The study found that 15 per cent of schizophrenia cases in men aged 16-49 could have been avoided by averting cannabis use disorder, compared to four per cent of females. For men aged 21-30, it could be as high as 25-30 per cent.
“Increases in the legalization of cannabis over the past few decades have made it one of the most frequently used psychoactive substances in the world, while also decreasing the public’s perception of its harm,” said lead author Carsten Hjorthøj in a press release. “This study adds to our growing understanding that cannabis use is not harmless, and that risks are not fixed at one point in time.”
The study’s findings are not surprising to David Saxby, an addiction counselor in Toronto.
“My experience has been, over the past 30 plus years of practice, that the younger an individual is when they commence regular marijuana use, the higher the risk of early onset schizophrenia,” Saxby said. “Males are more susceptible than females.”
Risks not appreciated
In 2019, Hammond and University of Waterloo colleague Samantha Goodman published a study showing that most cannabis users do not recognize the risk of psychosis and schizophrenia associated with cannabis use. The researchers polled over 15,000 Canadians, after cannabis was legalized, on the health effects of cannabis.
Only 21 per cent of daily users agreed that the regular use of marijuana can increase the risk of psychosis and schizophrenia.
“Lower perceived risk among consumers may reflect optimism bias, the belief that one’s health risk is lower than that of others, and an effort to minimize cognitive dissonance, in which consumers alter their health beliefs when it conflicts with their behavior,” Hammond and Goodman note.
Asked to comment on what the government is doing to inform Canadians about the risk of schizophrenia from cannabis use, a Health Canada spokesperson noted that “Health Canada has a number of web publications about the negative effects of cannabis on the brain.”
The spokesperson directed Canadian Affairs to the government’s Talk about Cannabis website page, which contains research about cannabis and its health effects.
There are nine rotating health warnings that appear on legal cannabis products in Canada. One of these says: “WARNING: Regular use of cannabis can increase the risk of psychosis and schizophrenia. Higher THC content can increase the risk of psychosis and schizophrenia.” (emphasis in the original)
“The dedicated health warning on Canadian packages is good, but insufficient on its own.” Hammond said.
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