Study: Chronic Marijuana Use May Increase Psychotic Experiences in Adolescents


A recent study published in the Journal of Child Psychology and Psychiatry showed a substantial increase in “psychotic-like experiences” in teenage users. The study, carried out by researchers in Canada, also reported adverse effects on cognitive development and increased symptoms depression.

The lead researcher, Josiane Bourque, a doctoral student at the University of Montréal’s Department of Psychiatry, explained “To clearly understand the impact of these results, it is essential to first define what psychotic-like experiences are: namely, experiences of perceptual aberration, ideas with unusual content and feelings of persecution.” reported:

One of the study’s objectives was to better understand the mechanisms by which marijuana use is associated with psychotic-like experiences. Bourque and her supervisor, Dr. Patricia Conrod at Sainte Justine University Hospital Research Centre hypothesized that impairments in cognitive development due to marijuana misuse might in turn exacerbate psychotic-like experiences.

This hypothesis was only partially confirmed, however. Among the different cognitive abilities evaluated, the development of inhibitory control was the only cognitive function negatively affected by an increase in marijuana use. Inhibitory control is the capacity to withhold or inhibit automatic behaviors in favor of a more contextually appropriate behavior. Dr. Conrod’s team has shown that this specific cognitive function is associated with risk for other forms of substance abuse and addiction.

Dr. Conrad has established a childhood drug use prevention program called “Preventure”. That program boasts a 33% reduction in adolescent marijuana use. She explained, “In future programs, it will be important to investigate whether this program and other similar targeted prevention programs can delay or prevent marijuana use in youth who suffer from psychotic-like experiences.”

The study was based on questionnaires filled out by approximately 4,000 Montreal youth between 13 and 16 years old. Every year between grade 7 to grade 11 the students fill out the questionnaire, which asses their substance use and psychiatric symptoms, along with cognitive tasks to evaluate their IQs.

“Our results show that while marijuana use is associated with a number of cognitive and mental health symptoms, only an increase in symptoms of depression – such as negative thoughts and low mood – could explain the relationship between marijuana use and increasing psychotic-like experiences in youth,” Bourque said. “Our findings confirm that becoming a more regular marijuana user during adolescence is, indeed, associated with a risk of psychotic symptoms. This is a major public-health concern for Canada.”


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