“Hasn’t pot always been considered harmful?
Not at all. Marijuana, the dried form of the plant Cannabis sativa, was used as an herbal remedy for centuries in China, the Middle East, and Asia. William O’Shaughnessy, a physician for the East India Tea Company, brought it west in the 1830s as a treatment for rheumatism, tetanus, and rabies. It was commonly prescribed as a pain reliever in the U.S. until the 1930s, when its growing popularity caused such concern that the newly founded Federal Bureau of Narcotics reclassified it as a narcotic. The bureau soon launched a decidedly unscientific campaign claiming that marijuana use provoked insanity, homicidal tendencies, and uncontrollable lust. The marijuana user, the bureau asserted, “becomes a fiend with savage or ‘caveman’ tendencies. His sex desires are aroused, and some of the most horrible crimes result.””

“Was there any evidence for such claims?
None; in fact, the American Medical Association argued against marijuana prohibition in the 1930s, citing its therapeutic potential. But the bureau made its case that marijuana was “dangerous for the mind and the body,” and the federal government outlawed its use in 1937. It wasn’t until the 1970s that a campaign began to restore marijuana’s therapeutic reputation, and in 1996 California became the first state to legalize cannabis for medicinal purposes. Psychiatrist Tod Mikuriya, a founding father in the medical marijuana movement, claimed that cannabis has none of the adverse side effects of opiates. “In fact,” he said, “it really enhances both quality of life and rehabilitation.””

More: http://theweek.com/article/index/236671/is-marijuana-bad-for-you