Teens who were heavy marijuana users had abnormal changes in their brains related to memory and performed poorly on memory tasks, reports a new study. The brain abnormalities and memory problems were observed in the subjects' early twenties, two years after they stopped smoking marijuana, possibly indicating long-term effects. Memory-related structures in their brains appeared to shrink. The younger drug abuse starts, the more abnormal the brain appeared. The marijuana-related brain abnormalities look similar to schizophrenia-related brain abnormalities.