BARBITURATE

any of a class of organic compounds used in medicine as sedatives (to produce a calming effect), as hypnotics (to produce sleep), or as an adjunct in anesthesia. The barbiturates are derivatives of barbituric acid (malonyl urea), which is formed from malonic acid and urea. Barbital was first synthesized in 1903, and phenobarbital became available in 1912. The barbiturates act by depressing the central nervous system; they act particularly on certain portions of the brain, though they tend to depress the functioning of all the body's tissues. Most of them exert a sedative effect in small doses and a hypnotic effect in larger doses. They are classified according to their duration of action. Long-acting barbiturates, such as barbital and phenobarbital, may last in their effects for as long as 24 hours and are used in conjunction with other drugs for the treatment of epilepsy, in which their prolonged depressant action helps prevent convulsions. Barbiturates of intermediate duration of action, such as amobarbital and butabarbital sodium, act for 6 to 12 hours and are used to relieve insomnia. Short-acting barbiturates, such as pentobarbital and secobarbital, are used to overcome difficulty in falling asleep. Ultrashort-acting barbiturates, such as thiopental sodium and thiamylal, are used intravenously to induce unconsciousness smoothly and rapidly in patients about to undergo surgery, after which gaseous anesthetics are used to maintain the unconscious state. The barbiturates have largely been replaced as sedatives by the benzodiazepines and other minor tranquilizers. Prolonged use of the barbiturates (especially secobarbital and pentobarbital) may in time cause the user to develop a tolerance to them and require amounts of the drug much larger than the original therapeutic dose. Denial of a barbiturate to the habitual user may precipitate a withdrawal syndrome that is indicative of physiological dependence on the drug. An overdose of barbiturates can result in coma and even death due to severe depression of the central nervous and respiratory systems.