opium alkaloid, one of several alkaloids isolated from the milky exudate of the unripe seed pods of Papaver somniferum, a species of poppy indigenous to the Near East. Two of the alkaloids, codeine and morphine, are used clinically for the relief of pain, but their use entails the risk of physical or psychological dependence. Morphine is the standard against which the analgesic effect of newer drugs for relief of pain is measured. The opium alkaloids and their semisynthetic derivatives, including heroin, act on the central nervous system, producing analgesia, change in mood, drowsiness, and mental slowness. The effects in a person who has pain are usually pleasant. Euphoria and pain-free sleep are not uncommon, but nausea and vomiting sometimes occur. In usual doses the analgesic effects are achieved without loss of consciousness. The opium alkaloids have several other effects on the various systems of the body: coughing is suppressed; the electrical activity pattern of the brain resembles that of sleep; the pupils constrict; respiration is depressed in rate, minute volume, and tidal exchange; the secretory activity and motility of the GI tract are diminished; and biliary and pancreatic secretions are reduced. The use of morphine as an antidiarrheal preceded its use as an analgesic by hundreds of years. Prepared in a tincture, it remains the most effective constipating agent available. Papaverine, another opium alkaloid, does not cause analgesia but is used clinically as a vasodilator.
