Young adults who used marijuana only recreationally showed significant abnormalities in two key brain regions that are important in emotion and motivation, scientists report. The study was a collaboration between Northwestern Medicine and Massachusetts General Hospital/Harvard Medical School.

This is the first study to show casual use of marijuana is related to major brain changes. It showed the degree of brain abnormalities in these regions is directly related to the number of joints a person smoked per week. The more joints a person smoked, the more abnormal the shape, volume and density of the brain regions.

“This study raises a strong challenge to the idea that casual marijuana use isn’t associated with bad consequences,” said corresponding and co-senior study author Hans Breiter, M.D. He is a professor of psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine and a psychiatrist at Northwestern Memorial Hospital.

“Some of these people only used marijuana to get high once or twice a week,” Breiter said. “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.”

The study will be published April 16 in the Journal of Neuroscience.

Scientists examined the  and the amygdala—key regions for emotion and motivation, and associated with addiction—in the brains of casual marijuana users and non-users. Researchers analyzed three measures: volume, shape and density of grey matter (i.e., where most cells are located in brain tissue) to obtain a comprehensive view of how each region was affected.

Both these regions in recreational pot users were abnormally altered for at least two of these structural measures. The degree of those alterations was directly related to how much marijuana the subjects used.

Of particular note, the nucleus acccumbens was abnormally large, and its alteration in size, shape and density was directly related to how many joints an individual smoked.

“One unique strength of this study is that we looked at the nucleus accumbens in three different ways to get a detailed and consistent picture of the problem,” said lead author Jodi Gilman, a researcher in the Massachusetts General Center for Addiction Medicine and an instructor in psychology at Harvard Medical School. “It allows a more nuanced picture of the results.”

– Northwestern University

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via Casual marijuana use linked to brain abnormalities in students.