New Study Shows Areas of the Brain Changed by “Casual” Marijuana Use

A new study that looked at recreational or “casual” marijuana use in young adults concluded that the drug can significantly change, and potentially even damage, the brain.

The study, published in the Journal of Neuroscience, is the first such study to link casual marijuana use to changes in the brain. Researchers looked at the brains of 40 young adults, ages 18 to 25 –half of the group reported smoking at least once a week, while the other half did not smoke at all.

The marijuana smokers were asked to track their cannabis use for 90 days. Then, both groups were given MRIs to look at their brains. When researchers compared the scans, they saw differences in the volume, shape, and density of the subjects’ brains, particularly the nucleus amygdala and the nucleus accumbens, which are two parts of the brain associated with emotion and motivation.

“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,” says Dr. Jodi Gilman, lead author and a researcher in the Massachusetts General Center for Addiction Medicine. “Those differences were exposure-dependent, meaning those who used more marijuana had greater abnormalities.”

Researchers also saw distinct differences in the amygdalas and accumbenses within the group of marijuana smokers, and linked the changes to the amount of recreational marijuana ingested. The median was six joints a week, but some of the recreational smokers reported only using marijuana once or twice a week, while some reported smoking more than 20 joints a week.

“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,” Breiter said. “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.”

Dr. Staci Gruber, director of the Cognitive and Clinical Neuroimaging Core at McLean Hospital in Boston and a professor of psychiatry at Harvard Medical School, has conducted numerous studies on marijuana use and brain function. “This data certainly confirms what others have reported with regard to changes in brain structure,” she said. “When we consider the findings of the Gilman … study with our own and other investigations of marijuana use, it’s clear that further investigation is warranted, specifically for individuals in emerging adulthood, as exposure during a period of developmental vulnerability may result in neurophysiologic changes which may have long-term implications.”

A number of new studies, particularly one from Duke University, point out that marijuana use in teenagers can be detrimental to critical brain development. The study from Duke University focused on residents of New Zealand, from childhood through age 38, and compared IQs between marijuana users and non-users.

“We found that people who began using marijuana in their teenage years and then continued to use marijuana for many years lost about 8 IQ points from childhood to adulthood,” says study author Madeline Meier, now a professor at Arizona State University, “whereas those who never used marijuana did not lose any IQ points.”

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