Marijuana is the most commonly abused illicit drug in the United States. A dry, shredded green/brown mix of flowers, stems, seeds, and leaves of the plant Cannabis sativa, it usually is smoked as a cigarette, or in a pipe. It also is smoked in blunts, which are cigars that have been emptied of tobacco and refilled with marijuana. It might also be mixed in food or brewed as a tea. As a more concentrated, resinous form it is called hashish and, as a sticky black liquid, hash oil. Marijuana smoke has a pungent and distinctive, usually sweet-and-sour odor.
The main active chemical in marijuana is THC (delta-9-tetrahydrocannabinol). The membranes of certain nerve cells in the brain contain protein receptors that bind to THC. Once securely in place, THC kicks off a series of cellular reactions that ultimately lead to the high that users experience when they smoke marijuana.
When marijuana is smoked, its effects begin immediately after the drug enters the brain and last from 1 to 3 hours. If marijuana is consumed in food or drink, the short-term effects begin more slowly, usually in ½ to 1 hour, and last longer, up to 4 hours. Smoking marijuana deposits several times more THC into the blood than does eating or drinking it.
Within a few minutes after smoking, the user's heart begins beating more rapidly, the bronchial passages relax and become enlarged, and blood vessels in the eyes expand, making the eyes look red.
As THC enters the brain, it causes a user to feel euphoric- or "high"-by acting in the brain's reward system, areas of the brain that respond to stimuli such as food and drink. THC activates the reward system by stimulating brain cells to release the chemical dopamine.
A marijuana user may experience pleasant sensations, colors and sounds may seem more intense, and time appears to pass very slowly. The user's mouth feels dry, and he or she may suddenly become very hungry and thirsty.
Someone who smokes marijuana regularly may have many of the same respiratory problems that tobacco smokers do, such as daily cough and phlegm production, more frequent acute chest illnesses, a heightened risk of lung infections, and a greater tendency toward obstructed airways.
Cancer of the respiratory tract and lungs may also be promoted by marijuana smoke. Marijuana's damage to short-term memory seems to occur because THC alters the way in which information is processed by the hippocampus, a brain area responsible for memory formation.