Heavy marijuana use during adolescence has now been linked to lower IQ scores later in life, according to a study published last month by the National Academy of Sciences.

The study, which tracked 1,037 subjects from birth to age 38 years, found that those who began smoking marijuana as teenagers and used it regularly throughout adulthood scored approximately 8 points lower on an IQ test than they had at age 13 years. In comparison, the IQ scores of non-users, as well as those who started using marijuana as adults, were stable. Small to medium declines in memory, processing speed, and executive function were also seen in regular users.

“We know that there are developmental changes occurring in the teen years and up through the early 20s, and the brain may be especially vulnerable during this time,” said Dr. Madeline Meier, a researcher at Duke University and lead author of the study, in an August 27 New York Times article.

The results of this study are in direct contrast with beliefs common among adolescents that marijuana use is harmless to health. “Adolescents are initiating cannabis use at younger ages, and more adolescents are using cannabis on a daily basis,” study authors said. “Findings are suggestive of a neurotoxic effect of cannabis on the adolescent brain and highlight the importance of prevention and policy efforts targeting adolescents.”

Although the authors ruled out several alternative explanations for the neuropsychological effects (such as hard drug use, alcohol dependence, and schizophrenia), they acknowledge that their results must be interpreted within the context of the study’s limitations. “There may be some ‘third’ variable that could account for the findings,” they said. “The data cannot reveal the mechanism underlying the associations between persistent cannabis dependence and neuropsychological decline.”

What do you think? Is there a disconnect between common beliefs about marijuana use and the reality of its long-term effects on health? PAR wants to hear from you, so leave a comment and join the conversation!