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Casual Marijuana Use Linked to Changes in Brain

The brain regions are tied to motivation, emotion and reward, researchers say

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The new study involved 40 people aged 18 to 25, all recruited from Boston-area colleges. Half said they used marijuana at least once a week, and the other half did not use the drug.

Psychiatric interviews revealed that the pot smokers did not meet criteria for drug dependence. For example, marijuana use did not interfere with their studies, work or other activities, and they had not needed to increase the amount they used to get the same high.

The researchers used MRI scans to study the participants' brains, focusing on the amygdala and nucleus accumbens. They analyzed three measures -- volume, shape and density of gray matter -- to gain a comprehensive view of how each region was affected.

The investigators found that the density of gray matter in the amygdala and nucleus accumbens was significantly increased in pot smokers compared with non-users, indicating abnormal growth of neurons in those locations of the brain, Gilman said.

As a result, both of the brain regions had become abnormally shaped, she said. The nucleus accumbens also was larger in pot smokers.

It also appears that the changes are more pronounced in people who report using marijuana more frequently during an average week.

"There was a direct, consistent relationship between how much marijuana they used and the abnormalities we saw," Breiter said.

The study found an association between marijuana use and brain anatomy, but it didn't prove a cause-and-effect link.

The next step in their research will be to see how these structural abnormalities relate to the behavior of a pot smoker, Gilman said.

"We think that abnormal neuronal growth is evidence that the brain is forming new pathways that could encourage future use of the drug," she said. "We do know there are clinically observable behavioral differences in people who smoke marijuana heavily -- for example, they have a hard time motivating themselves to accomplish goals. Maybe some of these brain changes can relate to some of the behavioral changes that have been observed clinically."

Breiter added that the pot being smoked by young people today is much more powerful than marijuana available to people in the 1960s. Today's marijuana contains much greater concentrations of THC, the primary psychoactive ingredient in pot.

"Levels of THC are about sevenfold what they used to be," he said. "That's a substantial change in the dosing of THC that these young people are getting. The experience of people in the '60s and '70s may not be the same experience as people today."

The study is published in the April 16 issue of The Journal of Neuroscience.

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