Casual Marijuana Use Physically Alters the Brain

A group of researchers from Massachusetts General Hospital, Harvard Medical School, and Northwestern University Feinberg School of Medicine have found significant changes in the size and shape of the emotion and reward processing regions of the brain in young adults who smoke marijuana at least once a week. The study, which was published by the Journal of Neuroscience, compared MRI images of individuals between the ages of 18 and 25 years who reportedly smoked marijuana one or more times a week with young adults who reported little or no history of marijuana use. According to researchers, the results raise concerns on the controversial issue of legalizing marijuana and the impact it could have on the nation’s youth and young adult population.

“I think the findings that there are observable differences in brain structure with marijuana even in these young adult recreational users indicate that there are significant effects of marijuana on the brain,” lead author Dr. Jodi Gilman stated in an interview with CNN. “Those differences were exposure-dependent, meaning those who used more marijuana had greater abnormalities.”

Scientists studied a total of 40 participants, 20 who smoked marijuana at least once a week and 20 who did not smoke any marijuana. Using MRIs, researchers compared the size, shape, and density of the amygdala and the nucleus accumbens—parts of the brain that respectively involve emotion and reward processing—between the two groups and discovered larger gray matter density, larger volume, and significant shape differences in those portions of the brain for casual marijuana smokers.

“This study suggests that even light to moderate recreational marijuana use can cause changes in brain anatomy,” stated Carl Lupic, PhD of the National Institute on Drug Abuse in an interview with the Society for Neuroscience. “These observations are particularly interesting because previous studies have focused primarily on the brains of heavy marijuana smokers, and have largely ignored the brains of casual users.”

The results of this study and similar studies could potentially dispel safety arguments in favor of the legalization of marijuana. “There’s a general idea out there that casual use of marijuana does not lead to bad effects, so we started out to investigate that very directly,” stated co-senior author Hans C. Breiter, MD. “This research with the other studies we have done have led me to be extremely concerned about the effects of marijuana in adolescents and young adults and to consider that we may need to be very careful about legalization policies and possibly consider how to prevent anyone under age 25 to 30 from using marijuana at all,” he said.