In 1972, after reviewing the scientific evidence, the National Commission on Marijuana and Drug Abuse concluded that while marijuana was not entirely safe, its dangers had been grossly overstated. Since then, researchers have conducted thousands of studies of humans, animals, and cell cultures. None reveal any findings dramatically different from those described by the National Commission in 1972. In 1995, based on thirty years of scientific research, editors of the British medical journal Lancet concluded that "the smoking of cannabis, even long term, is not harmful to health."
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 more recent 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 over 25 years that has never been supported by any scientific study.
Marijuana does produce immediate, temporary changes in thoughts, perceptions, and information processing. Short-term memory is the cognitive process most clearly affected by marijuana use. In laboratory studies, subjects under the influence of marijuana have no trouble remembering things they learned previously. However, they display diminished capacity to learn and recall new information. This diminishment only lasts for the duration of the intoxication. There is no evidence that heavy long-term marijuana use permanently impairs memory or other cognitive functions.
For 25 years, researchers have searched for a marijuana-induced amotivational syndrome and have failed to find it. People who are intoxicated constantly, regardless of the drug, are unlikely to be productive members of society. There is nothing about marijuana specifically that causes people to lose their drive and ambition. In laboratory studies, subjects given high doses of marijuana for several days or even several weeks exhibit no decrease in work motivation or productivity. Among working adults, marijuana users tend to earn higher wages than non-users and college students who use marijuana maintain the same grades as non-users. Among high school students, heavy use is associated with school failure, but school failure usually comes first.