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The common name of a small number of varieties of Cannabis sativa, or Indian hemp plant, which contain tetrahydrocannabinol (THC), a psychoactive drug.

Cannabis, in the form of marijuana, hashish (a dried resinous material that seeps from cannabis leaves and is more potent than marijuana), or other cannabinoids, is probably the most often used illegal substance in the world. In the United States, marijuana use became widespread among young people in the 1960s. By 1979, 68 percent of young adults between the ages of 18 and 25 had experimented with it at least once, and it was reported that as of the same year the total number of people in the U.S. who had tried the drug was 50 million. In the late 1980s, it was estimated that about 50 to 60 percent of people between the ages of 21 and 29 had tried marijuana at least once.

Marijuana and hashish are usually smoked, but may also be ingested orally, and are sometimes added to food or beverages. The psychoactive substance of cannabis is tetrahydrocannabinol, or THC, especially delta-9-tetrahydrocannabinol. Delta-9-THC can be synthesized, is known to affect the central nervous system, and has been legally used to treat side-effects of chemotherapy and weight loss in persons affected with AIDS. Other legal therapeutic uses of marijuana include the treatment of glaucoma and epilepsy.

The effects of cannabis use vary from individual to individual, depending on the physical and psychological condition of the user, the amount of THC consumed, and

A chemical present in marijuana has been shown to destroy brain cells in rats. (Photo by Tom McHugh. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.)

many other factors. Technically, marijuana is classified as a hallucinogen, but its effects are usually much milder than those of other drugs in this category, such as LSD, mescaline, and psilocybin. When it is inhaled through a marijuana cigarette, THC reaches its highest concentration in the blood within a half hour, and is absorbed by the brain and other organs, and can affect consciousness for several hours. THC can remain stored in body fat for several weeks. Marijuana users commonly experience feelings of euphoria, self-confidence, reduced inhibition, relaxation, and a floating sensation. Feelings of giddiness and mild feelings of paranoia are also common. Physiological effects include increases in pulse and heart rates, reddened eyes, dryness of the mouth, and an increased appetite. The initial euphoric feelings after ingesting marijuana are generally followed by sleepiness. Although marijuana has been known to produce psychological dependence, there is little tendency to become physically dependent on it, and withdrawal from the drug does not pose medical problems. Recently, receptors for THC have been discovered in the brain, together with a naturally-occurring substance—anandamide—that binds the chemical to its receptors and may be a neurotransmitter.

Documented negative effects of marijuana use include impairment in perception, sensory motor coordination, short-term memory, and panic attacks, and is also linked to impairment of the immune system, lowered testosterone levels in males, and chromosome damage. If taken by pregnant women, marijuana affects the developing fetus. Long-term marijuana smokers display similar respiratory dysfunctions as tobacco smokers In research on rats, THC has been found to destroy cells in the hippocampus, a part of the brain that is important in the formation of new memories. Psychologically, chronic use of marijuana has been associated with a loss of ambition known as amotivational syndrome. Authorities differ with respect to the physical and psychological risks of short-term and long-term use/abuse of cannabis. Current penalties for the illegal possession of marijuana, hashish, or other form of cannabis can be extremely severe.

See also Drugs/Drug abuse

Further Reading

Grinspoon, Lester. Marijuana Reconsidered. Oakland, CA: Quick American Archives, 1994.

Additional topics

Psychology EncyclopediaPsychological Dictionary: Ibn Bajjah (Abu-Bakr Muhammad ibn-Yahya ibn-al-Saʼigh, c.1106–38) Biography to Perception: cultural differences