Cannabis Use in Adolescence Associated With Poorer Mental Health Outcomes

A jar of marijuana
A jar of marijuana
These results provide strong evidence for the deleterious and prolonged effects of cannabis use in adolescence.

Prolonged cannabis use during adolescence is associated with poorer working memory, perceptual reasoning, and inhibitory control, according to study data published in The American Journal of Psychiatry. These effects may be more pronounced for cannabis compared with alcohol.

Researchers abstracted data from the Co-Venture study, a randomized controlled trial assessing the 5-year efficacy of a “personality-targeted” drug and alcohol prevention program. A population-based sample of 7th grade students from 31 schools in the Greater Montreal, Canada, area (n=3826) completed an annual substance use questionnaire for 4 consecutive years. During each survey period, participants also completed computerized tasks for cognition assessment and reported various demographic variables. Investigators used multi-level linear models to assess the influence of cannabis and alcohol consumption on 4 domains of cognition: spatial working memory, perceptual reasoning, delayed recall memory, and inhibitory control. A total of 3 multilevel linear models were developed: one for cannabis, one for alcohol, and one combining alcohol and cannabis. Baseline covariates, including socioeconomic status, were adjusted for in the effects model. Within-subject effects were interpreted as “neuroplastic,” or concurrent effects; time-lagged within-subject effects were interpreted as “neurotoxic,” or delayed effects.

The total study cohort was 47% girls and mean age was 12.7 (0.5). The cannabis-only model indicated that cannabis use predicted lower performance on working memory (P =.04), perceptual reasoning (P =.001), and inhibition (P <.01) over the 4-year period. A significant within-subject effect showed that any increase in cannabis use frequency was predictive of impairment in delayed recall memory (P <.01) in the same year. The alcohol-only model showed that average alcohol consumption over 4 years was associated with lower spatial working memory performance (P <.05), lower perceptual reasoning scores (P <.01), and more errors on the inhibitory control task (P <.01). No significant within-subject effects were observed for any of the cognitive domains, however. For the combined alcohol-cannabis model, substantial between-subject effects were observed for alcohol and perceptual reasoning (P =.03) and for cannabis and inhibitory control (P <.01). Lagged within-subject effects indicated that cannabis use frequency in a given year predicted lower inhibitory control (P <.01) and working memory (P <.09) a year later. A within-year increase in cannabis use frequency was associated with lower scores on the delayed recall memory task in the same year (P =.01).

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These results are consistent with a neuroplastic and neurotoxic effect of cannabis on inhibitory control and working memory, in that increased within-year use and time-lagged use were each associated with cognitive impairments.

As quantity or dose of cannabis was not assessed in this study, investigators cautioned that these data should be extrapolated with care. Still, such results provide strong evidence for the deleterious and prolonged effects of cannabis use in adolescence, which in some cases are greater than those observed for alcohol use.


Morin J-FG, Afzali MH, Bourque J, et al. A population-based analysis of the relationship between substance use and adolescent cognitive development [published online October 3, 2018]. Am J Psychiatry. doi:10.1176/appi.ajp.2018.18020202