Young adults who use marijuana only recreationally show significant abnormalities in two key brain regions that are important in emotion and motivation, scientists have found.
The study is the first to show that casual use of marijuana is related to major brain changes.
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, researchers said.
"This study raises a strong challenge to the idea that casual marijuana use isn't associated with bad consequences," said Hans Breiter, corresponding and co-senior author of the study published in the Journal of Neuroscience.
"Some of these people only used marijuana to get high once or twice a week," said Breiter, a professor of psychiatry and behavioural sciences at Northwestern University Feinberg School of Medicine and a psychiatrist at Northwestern Memorial Hospital.
"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," Breiter said.
Scientists examined the nucleus accumbens and the amygdala - key regions for emotion and motivation, and associated with addiction - in the brains of casual marijuana users and non-users.
Researchers analysed three measures: volume, shape and density of grey matter (ie, 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.
Through different methods of neuroimaging, scientists examined the brains of young adults, ages 18 to 25, from Boston-area colleges; 20 who smoked marijuana and 20 who didn't.
Each group had nine males and 11 females. The users underwent a psychiatric interview to confirm they were not dependent on marijuana. They did not meet criteria for abuse of any other illegal drugs during their lifetime.
The changes in brain structures indicate the marijuana users' brains are adapting to low-level exposure to marijuana, scientists said.
