Fact: None of the medical tests currently used to detect brain damage in humans have found harm from marijuana, even from long term high-dose use. An early study reported brain damage in rhesus monkeys after six months exposure to high concentrations of marijuana smoke. In a recent, more carefully conducted study, researchers found no evidence of brain abnormality in monkeys that were forced to inhale the equivalent of four to five marijuana cigarettes every day for a year. The claim that marijuana kills brain cells is based on a speculative report dating back a quarter of a century that has never been supported by any scientific study.