Depressants are drugs that lower the energy level of the nervous system, reducing sensitivity to outside stimulation and, in high doses, inducing sleep. Many depressants are used medically and socially and many are consumed illegally. In fact, depressants include some of the most popular drugs affecting mood.

At first glance, it may seem odd that people would want to take substances in order to depress their nervous systems. Who wants to feel groggy and depressed? Actually, depressants make people sleepy and stuporous only in high doses; in lower doses they often make people relaxed and happy. Alcohol is a good example. One or two drinks can make people feel cheerful, alert, and more alive, while three or four can make them drunk, with obvious symptoms of reduced brain function.

Scientists don't really know why low doses of depressants make people feel stimulated. One theory is that the first parts of the brain to be depressed are inhibitory centers that normally act to dampen mood. As the dose is increased, more and more parts of the nervous system are slowed down. Very high doses of depressants can cause coma, in which all consciousness is lost; a person in a coma is insensible to all input—even loud noise and intense pain. Still higher doses of these drugs can shut down the most vital centers in the brain, such as the one that controls respiration, resulting in a quick death from lack of oxygen.

Depressant drugs are more dangerous than stimulants because overdoses of them are apt to kill people by interfering with vital brain centers. Also, they produce a wider variety of effects, from relaxation and euphoria in low doses to unresponsive coma in very high doses.

In a kava ceremony on the island of Fiji, a leader of the ritual offers a bowl of the prepared drink to arriving guests. Kava is a natural depressant.

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