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Marijuana is a mixture of leaves, stems and flowers of the Indian hemp plant Cannabis sativa, and is smoked or eaten for its hallucinogenic and pleasure-giving effects.

The psychoactive ingredient of marijuana, tetrahydrocannabinol (THC), is concentrated in the flowers. Hashish, a drug prepared from the plant resin, has about eight times more THC than marijuana. Marijuana grows throughout temperate regions, with the more potent varieties produced in hot, dry, upland climates. Except for limited medical purposes, cultivating marijuana is illegal in all but a few countries.

Known in Central Asia and China as early as 3,000 B.C., marijuana was used as a folk medicine. About 1900 A.D. it started to be used as a pleasure-inducing drug, and by the 1960's and '70's its use was widespread among students, becoming, after alcohol, the second most popular drug. Although marijuana has not been proven to be physically addicting, and no physical withdrawl symptoms occur when its use is discontinued, psychological dependence does develop. Many users describe two phases of marijuana intoxication: initial stimulation, giddiness and euphoria, followed by sedation and pleasant tranquility. Mood changes are often accompanied by altered perceptions of time and space and of one's bodily dimensions. The thinking process becomes disrupted by fragmented ideas and memories. Many users report increased appetite, heightened sensory awareness and pleasure. Negative effects can include confusion, acute panic reactions, anxiety attacks, fear, a sense of helplessness and loss of self-control.

Chronic marijuana users are said to develop an "amotivational syndrome" characterized by passivity, decreased motivation and preoccupation with drug taking. The relationship of this syndrome to marijuana use, however, has not been established. Like alcohol intoxication, marijuana intoxication impairs reading comprehension, memory, speech, problem-solving ability and reaction time. The effects of long-term use are unknown. Consistent evidence that marijuana induces or causes brain damage does not exist. Medical researchers are studying the effectiveness of the drug in relieving some of the symptoms of glaucoma and in treating the nausea induced by cancer chemotherapy and radiation treatments.