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Marijuana, long considered a benign illicit substance, has recently been associated with drug dependence, amotivational states, and, possibly, psychosis. Researchers in Australia used high-resolution MRI to examine cannabinoid-receptor–rich regions in the amygdala and hippocampus of 15 heavy cannabis users (mean daily use, 5–7 joints; mean duration of regular use, 20 years) and 16 nonusers. The participants, all men, did not meet criteria for DSM disorders; cannabis was the primary drug of abuse among users; and alcohol use was similar between groups.
Left and right hippocampus volumes were 12% smaller in cannabis users than in controls (effect size, 1.2); left and right amygdala volumes were 6% and 8% smaller, respectively (effect sizes, 0.…