Impact of Cannabis Use on the Adolescent Brain


Even moderate cannabis use impacts cognitive functioning!

A recent study from the University of Colorado School of Medicine investigated the impact of marijuana use on cognitive function in teen siblings. The study looked at siblings in order to control for a number of familial factors and to investigate whether genetic factors explain a shared risk for worse cognitive functioning.

It is already commonly known that marijuana can have adverse effects, especially on attention, memory and learning, as well as on physical health (respiratory and heart diseases, cancer, pregnancy issues, etc.).

Source: Nora D et al., Weiss, PhD, N Engl J Med (2014).

The study included 1192 teens from 596 families, with 64% of the teens being males. Data collection took place throughout 2001-2006 and 2008-2013. Cannabis use was measured using interviews, and cognitive functioning was primarily measured via verbal memory, amongst other neuropsychological tests.

The results showed that:

  • A greater frequency and earlier onset of regular cannabis use was associated with poorer cognitive performance.
  • After the authors accounted for familial factors (shared by siblings) and alcohol use, poorer verbal memory performance was still associated with higher frequency of cannabis use.

These results indicate that moderate cannabis use throughout adolescence may have adverse effects on cognitive functioning, particularly in verbal memory, which cannot be explained by familial factors.

Early use of cannabis impacts girls’ working memory more than boys’!

An additional recent study from the University of Montreal investigated the impact of cannabis use on working memory and academic success in adolescents, to compare its effect on boys and girls.

The authors carried out a 5-year study on 3826 students from 31 different high schools in Montreal. The neuropsychological tests measured working memory, recall memory, perceptual reasoning and inhibitory control. All of the students were tested annually throughout the 5 years in order to analyse their development.

The results showed:

  • An overall gradual increase in marijuana and alcohol use amongst both boys and girls, and by the time students had reached grade 5, almost 80% had used cannabis.
  • Alcohol consumption was not associated with a reduction in cognitive performance.
  • However, cannabis use decreased academic performance in both boys and girls, and working memory was greatly affected for those who used cannabis earlier (at the start of high school).
  • Girls were more affected by the reduced cognitive performance than boys, and girls who used cannabis earlier were more affected than girls who used it later (end of high school). It is known that girls finish their brain development earlier than boys, with the last neural region to develop being the prefrontal cortex (where working memory takes place). Therefore, it is likely that girls using cannabis during this time may affect the maturation of their prefrontal cortex.
Source: The science of psychotherapy. Prefrontal Cortex. Dahlitz (2017).

The authors of the study explained that although there are laws prohibiting the use of marijuana before the ages of 18 or 21, this will not stop adolescents from using it. They, therefore, want to find a way to prevent marijuana use as effectively as possible, in order to decrease the risks of cognitive decline.


  1. Jarrod M Ellingson et al. “Familial factors may not explain the effect of moderate‐to‐heavy cannabis use on cognitive functioning in adolescents: a sibling‐comparison study”. Addiction. University of Colorado. (September 2020)
  2. Patricia Conrod et al. “Cognitive Function Impairments Linked to Alcohol and Cannabis Use During Adolescence: A Study of Gender Differences”.  Frontiers in Human Neuroscience. University of Montreal. (April 2020)

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