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Ian Gold is associate professor of Philosophy and Psychiatry at McGill University. Joel Gold is Clinical associate professor of Psychiatry at the NYU School of Medicine. They are the authors of Suspicious Minds: How Culture Shapes Madness.

The legalization of cannabis in Canada marks the beginning of a social experiment that raises a momentous health question. Cannabis use is linked to an increased risk of developing psychosis – a family of psychiatric disorders characterized by delusions, auditory hallucinations and bizarre thinking. Schizophrenia is the best known and most devastating of these and is among the top 15 causes of disability worldwide. Schizophrenia typically develops in late adolescence or early adulthood, and because it often persists for decades, it brings with it untold suffering to survivors and their families. In addition, when last investigated in 2004, the annual economic burden of schizophrenia in Canada was found to be nearly $7-billion in health care costs and loss of productivity.

Psychosis is arguably the most serious health problem associated with cannabis use and the evidence for the association is strong. A study published this year surveying nearly 100 possible factors associated with psychosis identified cannabis use as one of four most definitively linked to the disorder. One seminal, longitudinal study that followed 45,000 Swedish conscripts over 15 years found a six-fold increase in the risk of developing schizophrenia in those who consumed high amounts of cannabis. A more recent follow-up study of the same group showed that even moderate use (11-50 times) doubled one’s risk. While the numbers might vary from study to study, the results are unequivocal: using cannabis is a risk factor for later psychosis. And the earlier the use, the greater the risk.

Another study found that the risk of later psychosis for 15-year-old users was three times as high as for 18-year-olds. Further, there are many varieties of cannabis, and the greater the content of THC (tetrahydrocannabinol) – the psychoactive agent that provides the “high” – in the material consumed, the greater the risk of illness. Since levels of THC in marijuana have increased significantly in recent years, the risk of psychosis has increased in the population, even if the frequency of use has not.

Nevertheless, the attention given to psychosis in the debate about legalization has been shockingly inadequate. This is particularly egregious given that young people are most vulnerable to the effects of cannabis but, as last year’s study conducted by the Canadian Centre on Substance Abuse and Addiction revealed, Canadian youth are confused about the dangers of its use.

Both research and education are required to begin to address this inattention. With respect to research, the advent of legal cannabis places data obligations on government public health agencies. First, more research must be carried out to better understand the ways in which cannabis and psychosis interact. The Canadian context may offer a unique research opportunity. Where cannabis sales are controlled exclusively by the province, purchasing data could be shared with mental health epidemiologists to develop our understanding of the patterns of use and illness. Beyond epidemiology, basic neuroscience as well as psychosocial research is needed to illuminate the many causal, behavioural and cultural phenomena that play a role in cannabis use.

Second, the incidence of psychosis in Canada’s youth should be monitored in order to identify trends that require intervention. Sadly, our data as to the number of people experiencing mental illnesses in general is poor, but that should not deter epidemiologists and clinicians from establishing new monitoring systems and research activities that can directly influence policy decisions.

Public education is the other crucial initiative. While we applaud the requirements of thorough product warning labels, much wider public information campaigns targeting adolescents and young adults are required so that they can make fully-informed decisions as to the risk they take when choosing to use cannabis. In our experience many young people who would not risk their health with a puff of tobacco appear to believe that cannabis use is risk-free. Anti-smoking campaigns have been extremely successful. A Canadian cannabis and psychosis campaign should follow that lead.

For all the social good that legal cannabis might bring, Canadians must be fully informed about the risks; the Schizophrenia Society of Canada’s Explore the Link project is a good place to start. Today should be the day when we begin to educate young Canadians about the effects of cannabis on psychosis. Success in this endeavour would count among the greatest boons of legalization.

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