Marijuana Use May Shrink the Brain

Smoking Pot Could Have Lasting Effects on the Brain

Medically Reviewed by Louise Chang, MD on June 02, 2008
From the WebMD Archives

June 2, 2008 -- Long-term marijuana use may actually shrink certain parts of the brain and have lasting effects on mental health.

A new study shows heavy marijuana use over several years was associated with structural differences in at least two different regions of the brain, the hippocampus and amygdala.

Researchers found that the hippocampus, which is thought to regulate memory, was an average of 12% smaller among marijuana users, compared with people who didn't smoke pot. The amygdala, involved in emotion and memory, was an average of 7% smaller.

The study also suggests that long-term marijuana users were more likely to report symptoms associated with mental disorders, although the strength of their symptoms didn't meet the criteria for diagnosis of mental illness.

Smoking Pot May Go to Your Head

Researchers say there is conflicting evidence regarding the long-term effects of marijuana use on the brain.

"Although growing literature suggests that long-term cannabis use is associated with a wide range of adverse health consequences, many people in the community, as well as cannabis users themselves, believe that cannabis is relatively harmless and should be legally available," writes researcher Murat Yucel, PhD, of ORYGEN Research Centre the University of Melbourne in Australia, and colleagues in the Archives of General Psychiatry.

"With nearly 15 million Americans using cannabis in a given month, 3.4 million using cannabis daily for 12 months or more and 2.1 million commencing use every year, there is a clear need to conduct robust investigations that elucidate the long-term sequelae of long-term cannabis use," they write.

In the study, researchers used high-resolution magnetic resonance imaging to compare the brain structure of 15 men who smoked more than five joints of marijuana daily for more than 10 years with images from 16 men who did not smoke pot.

The participants also took a verbal memory test and were evaluated for symptoms of mental disorders.

The results showed men who smoked pot regularly had significantly lower brain tissue volumes in the hippocampus and amygdala areas, as well as more symptoms of mental disorders.

Researchers say marijuana users also performed significantly worse on the verbal learning test, but these differences did not correlate with brain volumes in either group.

"There is ongoing controversy concerning the long-term effects of cannabis on the brain," write the researchers. "Although modest use may not lead to significant neurotoxic effects, these results suggest that heavy daily use might indeed be toxic to human brain tissue."

Show Sources


Yucel, M. Archives of General Psychiatry, June 2008; vol 65: pp 694-701.

News release, American Medical Association.

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