chemical substance used to reduce tension and anxiety and induce calm (sedative effect) or to induce sleep (hypnotic effect). Most such drugs exert a quieting or calming effect at low doses and a sleep-inducing effect in larger doses. Sedative-hypnotic drugs tend to depress the central nervous system. Since these actions can be obtained with other drugs, such as opiates, the distinctive characteristic of sedative-hypnotics is their selective ability to achieve their effects without affecting mood or reducing sensitivity to pain.

For centuries alcohol and opium were the only drugs available that had sedative-hypnotic effects. chloral hydrate, a derivative of ethyl alcohol, was introduced in 1869 as the first synthetic sedative-hypnotic, and a more important drug, barbital, was synthesized in 1903. phenobarbital became available in 1912 and was followed, during the next 20 years, by a long series of other barbiturates (barbiturate). In the mid-20th century new types of sedative-hypnotic drugs were synthesized, chief among them the benzodiazepines (the so-called minor tranquilizers).

Barbiturates were extensively used as “sleeping pills” throughout the first half of the 20th century. Among the most commonly prescribed kinds were phenobarbital, secobarbital (marketed under Seconal and other trade names), amobarbital (Amytal), and pentobarbital (Nembutal). When taken in high enough doses, these drugs are capable of producing a deep unconsciousness that makes them useful as general anesthetics. In still higher doses, however, they depress the central nervous and respiratory systems to the point of coma, respiratory failure, and death. Additionally, the prolonged use of barbiturates for relief of insomnia leads to tolerance, in which the user requires amounts of the drug much in excess of the initial therapeutic dose, and to addiction, in which denial of the drug precipitates withdrawal, as indicated by such symptoms as restlessness, anxiety, weakness, insomnia, nausea, and convulsions.

Because of these health risks, the barbiturates were gradually supplanted by the benzodiazepines beginning in the 1960s. The latter are more effective in relieving anxiety than in inducing sleep, but they are superior to barbiturates because of the reduced dangers they present of tolerance and addiction and because they are much less likely to injuriously depress the central nervous system when used at high doses. They also require a much smaller dosage than do barbiturates to achieve their effects. The benzodiazepines include chlordiazepoxide (Librium), diazepam (Valium), alprazolam (Xanax), oxazepam (Serax), and triazolam (Halcion). They are, however, intended only for short- or medium-term use, since the body does develop a tolerance to them and withdrawal symptoms (anxiety, restlessness, and so on) develop even in those who have used the drugs for only four to six weeks. The benzodiazepines are thought to accomplish their effect within the brain by facilitating the action of the neurotransmitter gamma-aminobutyric acid, which is known to inhibit anxiety.

Antipsychotic drugs (major tranquilizers), tricyclic antidepressants, and antihistamines (antihistamine) can also induce drowsiness, though this is not their primary function. Most over-the-counter sleeping aids use antihistamines as their active ingredient.