Adolescent cannabis use showed both immediate and lasting (neurotoxic) effects on inhibitory control, working memory, memory recall, and perceptual reasoning, with effects more pronounced than alcohol and independent of it.
Parents, adolescent health clinicians, educators, and policymakers.
Cannabis worse than alcohol for cognition
What the researchers found
Cannabis showed lagged (neurotoxic) effects on inhibitory control and working memory, and concurrent effects on delayed memory recall and perceptual reasoning. These effects were independent of alcohol use. Alcohol showed vulnerability effects but no evidence of lasting (lagged) cognitive impact. Some evidence of developmental sensitivity was found for cannabis effects.
Why it matters
Published in the American Journal of Psychiatry, this study separates four theoretical models of how substances affect cognition and finds that cannabis uniquely shows the neurotoxicity pattern (lasting effects from prior use), while alcohol does not.
The numbers in context
3,826 students from 31 schools; 4 annual assessments; 4 cognitive domains tested; cannabis showed significant lagged effects on inhibitory control and working memory; cannabis effects independent of alcohol effects.
How the study worked
Population-based longitudinal study of 3,826 seventh-graders from 31 schools in Greater Montreal, assessed annually for 4 years on substance use and four cognitive domains using computerized testing. Multilevel regression models tested vulnerability, concurrent, and lagged effects.
What this study cannot tell us
Self-reported substance use. Cannabis potency not assessed. Four-year follow-up may not capture long-term recovery. Greater Montreal population may not generalize globally. Computerized cognitive testing may miss real-world functional impairment.
How to read the evidence
Strong: large population-based longitudinal study with annual assessments and sophisticated modeling, published in Am J Psychiatry.
When this study was published
Published in 2019.
The bigger picture
This is among the strongest evidence that adolescent cannabis is worse for cognition than adolescent alcohol, a finding that challenges the popular perception of cannabis as the "safer" substance for teenagers.
Questions still open
- Do the cognitive effects persist beyond adolescence? Would the effects be greater with today's higher-potency cannabis? Can early cognitive effects predict later psychiatric outcomes?
Common questions
Is cannabis or alcohol worse for teenage brains?
Are the cognitive effects permanent?
Read the original research
A Population-Based Analysis of the Relationship Between Substance Use and Adolescent Cognitive Development.
The American journal of psychiatry, 176(2), 98-106
Citation
Morin, Jean-François G; Afzali, Mohammad H; Bourque, Josiane; Stewart, Sherry H; Séguin, Jean R; O'Leary-Barrett, Maeve; Conrod, Patricia J. (2019). A Population-Based Analysis of the Relationship Between Substance Use and Adolescent Cognitive Development.. The American journal of psychiatry, 176(2), 98-106. https://doi.org/10.1176/appi.ajp.2018.18020202
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