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Tuesday, June 12 2018

Cannabis and Vulnerability to Psychosis

A study led by a research team at CHU Sainte-Justine confirms that cannabis use precedes and predicts psychotic symptoms in a population-based study of Canadian youth

MONTREAL, June 12, 2018 – The relationship between cannabis use and psychosis has been reported in previous studies, but few have been able to demonstrate that there is any causal effect of cannabis use on mental health symptoms. Researchers at CHU Sainte-Justine and the University of Montreal used a unique longitudinal sample of Montreal high school students to examine the direction of the relationship between these two health concerns. Using advanced statistical modeling, they showed for the first time, that onset or increase in cannabis use behaviour in any given year during adolescence predicted increases in psychotic symptoms that year and one year later. These relationships could not be accounted for by any pre-existing common vulnerability to cannabis use or psychosis-proneness. The results of this study were published today in the scientific journal JAMA Psychiatry.

“These findings are novel because they demonstrate that the link between cannabis and vulnerability to psychosis can be observed at a population level,” said the study’s senior author, Patricia Conrod, Researcher at CHU Sainte-Justine and Professor at University of Montreal’s Department of Psychiatry. “Many people think that cannabis only causes psychotic symptoms in individuals at risk for psychosis, but this study challenges that view.”

Lead author and PhD student at University of Montreal, Josiane Bourque, performed this analysis as part of her doctoral thesis. “It is difficult to demonstrate causality between two events in human research because experimental designs cannot be used to test the effect of cannabis on mental health outcomes. Therefore, we used longitudinal observational data and a statistical model that allowed us to dissociate changes within a person’s own trajectory from overall differences between individuals. Our analyses showed that cannabis use always preceded an increase in psychotic symptoms, but psychotic symptoms rarely preceded increases in cannabis use.” The authors concluded that the findings are consistent with a causal hypothesis.

What’s Next

In any given year, approximately 26-36% of North American youth between 15-24 years of age report using cannabis in the past year. “These findings have important public health repercussions because a significant portion of the youth population is exposing itself to an important public health risk, and might not be aware of such risk,” says Dr. Conrod. “Youth cannabis prevention must be a priority for governments taking on the responsibility of cannabis regulation.”

“While we only showed an effect on psychotic symptoms and not on transition to major psychiatric conditions, elevated and persistent psychotic symptoms are associated with an increased risk for psychotic disorders, such as acute psychotic episodes, bipolar disorder and schizophrenia,” explains Josiane Bourque. “These findings might suggest that effective cannabis prevention could potentially reduce risk for major mental health conditions in the population, but this must be further investigated.”

Dr. Conrod is optimistic about one thing, however: school-based prevention programs can be effective in helping young people to delay or reduce their cannabis use. “We now need effective implementation strategies to help educational and health professionals to implement and sustain evidence-based cannabis prevention in their communities.”

A Large Youth Cohort From Montreal

The study’s results are based on CIHR-funded Co-Venture project, a cohort of approximately 4,000 adolescents aged 13 years old. The sample includes 76% of all Grade 7 students attending 31 secondary schools in the greater Montreal area in 2012-2013, representing 15% of all schools in the area and each of their respective school districts in size and sociodemographics. These teens were followed annually from Grade 7 to Grade 11. Every year they filled out computerized questionnaires to assess substance use and psychotic symptoms.

To do their study, the research team first accounted for general relationships between overall tendency to be a cannabis user in adolescence and to report psychotic symptoms. Then, they examined how any further increase in an individual’s cannabis use (or psychotic symptoms) predicted and preceded further increases in psychotic symptoms (or cannabis use). The analytic strategy can be best understood using the following diagram:

PS: psychotic symptoms
CAN: cannabis use
Numbers refer to age


About this study

Bourque, J., Afzali, M.H., Conrod, P.J. “Association of cannabis use and adolescent psychotic symptoms” published in the Journal of the American Medical Association – Psychiatry, June 6, 2018. The first author is Josiane Bourque, a doctoral student under Patricia Conrod’s supervision. Senior author is Patricia Conrod, PhD, Researcher and Director of the Venture laboratory at CHU Sainte-Justine, Full Professor in the Department of Psychiatry of Université de Montréal and holder of the Chair Dr Julien/Fondation Marcelle and Jean Coutu in Social and Community Pediatrics of Université de Montréal.

This study was funded by a Canadian Institutes of Health Research (CIHR) doctoral fellowship to Josiane Bourque, a Fonds de recherche du Québec – Santé senior investigator award to Patricia Conrod, and two CIHR grants to Patricia Conrod and co-investigators (FRN114887 and SHI155406).

About the CHU Sainte-Justine Research Center

CHU Sainte-Justine Research Center is a leading mother-child research institution affiliated with Université de Montréal. It brings together more than 200 research investigators, including over 90 clinician-scientists, as well as 450 graduate and postgraduate students focused on finding innovative prevention means, faster and less invasive treatments, as well as personalized approaches to medicine. The Center is part of CHU Sainte-Justine, which is the largest mother-child center in Canada and second pediatric center in North America. More on research.chusj.org

CHU Sainte-Justine

Maude Hoffmann
Communications, CHU Sainte-Justine Research Center

Media contact:
Chantale Laberge
CHU Sainte-Justine
514-345-4931, ext. 2746

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Updated on 6/12/2018
Created on 6/12/2018
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