Recreational marijuana use has been linked to significant abnormalities in two regions of the brain, according to a study conducted by Northwestern Medicine and Massachusetts General Hospital/Harvard Medical School.

The study, published Wednesday in the Journal of Neuroscience, examined the number of joints individuals smoked per week and compared it to the significance of their brain abnormalities. Researchers found that the greater amount smoked per week, the more abnormal the brain regions were in shape, volume and density.

“Some of these people only used marijuana to get high once or twice a week,” said Feinberg Prof. Dr. Hans Breiter, a co-senior author of the study, in a news release. “People think a little recreational use shouldn’t cause a problem, if someone is doing OK with work or school. Our data directly says this is not the case.”

During the study, researchers looked into three separate categories — including the volume, shape and density of grey matter within specific regions of the brain — in order to gain a comprehensive measure of the extent of the expressed abnormalities. The study ultimately found a direct correlation between abnormalities in at least two of the measures and the amount of recreational marijuana individuals smoked. Jody Gilman, lead author of the study, said the study is unique in its detail and results.

“This study raises a strong challenge to the idea that casual marijuana use isn’t associated with bad consequences,” Breiter said in the release.

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