New study finds heavy marijuana use harms brain

Frequent marijuana use has a powerful impact on the human brain's intellectual functions, particularly among people who start using it while young, scientists have found. Researchers report that heavy marijuana use is linked to adverse changes in key brain regions -- those associated with reward, decision-making and motivation. Heavy marijuana use was described as four times a week for at least six months, according to a report in the Proceedings of the National Academy of Sciences.