Does Marijuana Affect Your Brain Years After Use?

The effects of marijuana on cognition and brain functioning are a highly contentious issue. Of particular concern is how long the possible negative effects of cannabis use may last. Do they lead to lifelong problems? Or do they simply fade away soon after the plant's psychoactive effects wear off? How do they affect brain development in teenagers? Well, a 2021 review of meta-analyses out of the University of Montreal in Canada sheds some new light on these perplexing and controversial issues.

According to the researchers, who looked at 10 different meta-analyses on marijuana's effects on the brains of both adolescents and adults, verbal learning and memory are the cognitive functions most impaired by cannabis use. More importantly, though, the meta-review concluded that these impairments can linger for weeks, meaning that "residual effects" of cannabis use do last well beyond the timeframe of acute use.

Co-author Dr. Alexandre Dumais, associate clinical professor of psychiatry at the University of Montreal, said that "Our study enabled us to highlight several areas of cognition impaired by cannabis use, including problems concentrating and difficulties remembering and learning, which may have considerable impact on users' daily lives" (via Neuroscience News).

The study also lends more weight to a body of evidence that shows that cannabis use may affect the development of the brain in teenagers. Using an MRI scanner, psychologists at the University of Vermont showed in a 2021 study that marijuana use was associated with the thinning of the pre-frontal cortex in adolescents. The pre-frontal cortex is involved in decision-making and other skills, leading the researchers to conclude that cannabis consumption may impair teenagers' ability to refrain from impulsive behavior (via Science for Students).

According to a 2020 review article published in the journal Frontiers of Psychiatry, adolescent cannabis use may contribute to "adverse neurocognitive effects that appear to show a level of permanency into adulthood".

Cannabis use may not lead to the same long-term effects in adults that they do in teenagers, however. According to the authors of the meta-review out of the University of Montreal, some residual deficits were only found in frequent adolescent users and were absent in chronic adult users.

Ultimately, the National Institutes of Health notes that more research is needed before we know for sure how long the effects of marijuana really last.