Other studies have revealed brain changes among heavy marijuana users, but this research is believed to be the first to demonstrate such abnormalities in young, casual smokers. The study found volume, shape and density changes in two crucial brain areas — the nucleus accumbens and the amygdala — involved with emotion and motivation and some types of mental illness. Scans revealed that the nucleus accumbens was larger in marijuana users, compared with nonusers, and its alteration was directly related to how much the person smoked. "There have been a growing number of studies that suggest that marijuana use in emerging adults is associated with differences in brain structure and cognitive abilities," said Gruber, also the director of Cognitive and Clinical Neuroimaging Core at McLean Hospital outside Boston. "Just casual use appears to create changes in the brain in areas you don't want to change," said Hans Breiter, a psychiatrist and mathematician at the Northwestern University Feinberg School of Medicine in Chicago, who led the new study.
– Even casual use of marijuana could be messing up young people's brains at a time of life when they need to make major decisions, a new study finds. Researchers say that people aged 18 to 25 who used marijuana at least once a week were found to have abnormalities in the parts of the brain linked to emotion, decision-making, and motivation, the Boston Globe reports. While it's not clear whether the changes are linked to decreased function, "this is a part of the brain you do not want to mess around with," the lead researcher warns. The casual tokers had an average of six joints a week, and the changes appeared to be more pronounced the more that was smoked. The changes in young brains are reason to worry, the lead researcher says, because "this is when you are making major decisions in your life, when you are choosing a major, starting a career, making long-lasting friendships and relationships." A neuropharmacologist not involved with the study says that while he doesn't doubt the results, he is wary of marijuana studies like this one that were funded by federal agencies tasked with limiting drug use. "If it were my child, even with this study, I'm more comfortable with young people having a marijuana habit than drinking regularly," he tells USA Today.