Stronger Cannabis Linked to Higher Psychosis Risk in Teens

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A new study entitled Age-dependent association of cannabis use with risk of psychotic disorder published in the journal Psychological Medicine estimates that teens using cannabis are at 11 times higher risk of developing a psychotic disorder compared to teens not using cannabis.

This finding suggests that the association between cannabis and psychotic disorders may be stronger than indicated by previous research, which has relied largely on older data when cannabis was less potent than today. For context, the average THC (tetrahydrocannabinol) potency of cannabis in Canada has increased from roughly 1 per cent in 1980 to 20 per cent in 2018.

Researchers from the Centre for Addiction and Mental Health (CAMH), the University of Toronto, and ICES linked recent population-based survey data from over 11,000 youth in Ontario, Canada, to health service use records including hospitalizations, emergency department (ED) visits, and outpatient visits.

The study is the first to show an age-dependent association between self-reported cannabis use and subsequent psychotic disorder diagnosis, which adds to a growing body of research on the mental health risks associated with cannabis.

“We found a very strong association between cannabis use and risk of psychotic disorder in adolescence. Surprisingly, we didn’t find evidence of association in young adulthood,” says lead author Dr. André McDonald, who conducted the study at ICES as part of his PhD training at the Institute for Mental Health Policy Research at CAMH and the University of Toronto. McDonald is now a postdoctoral fellow at the Peter Boris Centre for Addictions Research and the Michael G. DeGroote Centre for Medicinal Cannabis Research at McMaster University. “These findings are consistent with the neurodevelopmental theory that teens are especially vulnerable to the effects of cannabis.”

Of the teens who were hospitalized or visited an ED for a psychotic disorder, roughly 5 in 6 had previously reported cannabis use. McDonald points out that, “the vast majority of teens who use cannabis will not develop a psychotic disorder, but according to these data, most teens who are diagnosed with a psychotic disorder likely have a history of cannabis use.”

The researchers could not completely rule out reverse causation, in that teens with psychotic symptoms may have been self-medicating with cannabis before receiving a clinical diagnosis. They also could not account for potentially important factors such as genetics and history of trauma. These limitations make it impossible to say definitively that teen cannabis use causes psychotic disorders. The authors also note that their estimates are only approximate, suggesting that further studies with larger samples are required.

Nevertheless, the findings add to worries about early cannabis use, particularly in the wake of legalization.

On March 22, 2024, the federal Minister of Health shared the final report of a legislative review which focused on progress made towards achieving the objectives of the Cannabis Act, and identifying priority areas for improving the functioning of this legislation. That report included a number of recommendations for limiting cannabis harms, including:

• Reinstating health warning messages that pertain to serious cannabis-related mental health risks including psychosis and schizophrenia;
• Supporting  the development and implementation of evidence-based school prevention programs and other interventions to reduce the prevalence of youth cannabis use;
• Taking steps to mitigate the risks associated with cannabis products that contain higher quantities or concentrations of THC; and 
• Taking action against producers and distributors who don’t comply with rules that protect youth

“The recommendations coming out of the Cannabis Act legislative review seem even more urgent given this latest research,” says Dr. Leslie Buckley, Chief of the Addictions Division at CAMH. “We see firsthand the devastating and irreversible effects of cannabis use on young people’s brains. Health care providers, educators, legislators, parents and youth all have an important role to play in mitigating these impacts through education, policy and prevention strategies.”

McDonald adds, “Canadian youth are among the heaviest users of cannabis in the world. If we follow the precautionary principle, the bottom line is that more needs to be done to prevent early cannabis use.”

Reference: McDonald AJ, Kurdyak P, Rehm J, Roerecke M, Bondy SJ. Age-dependent association of cannabis use with risk of psychotic disorder. Psychol Med. 2024:1-11. doi: 10.1017/S0033291724000990

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