alkaloid, any of a class of organic compounds composed of carbon, hydrogen, nitrogen, and usually oxygen that are often derived from plants. Although the name means alkalilike, some alkaloids do not exhibit alkaline properties. Many alkaloids, though poisons, have physiological effects that render them valuable as medicines. For example, curarine, found in the deadly extract curare, is a powerful muscle relaxant; atropine is used to dilate the pupils of the eyes; and physostigmine is a specific for certain muscular diseases. Narcotic alkaloids used in medicine include morphine and codeine for the relief of pain and cocaine as a local anesthetic. Other common alkaloids include quinine, caffeine, nicotine, strychnine, serotonin, and LSD. Aconitine is the alkaloid of aconite. Cinchonine and quinine are derived from cinchona, coniine is found in poison hemlock, and reserpine is an extract of rauwolfia roots. Emetine is an alkaloid of ipecac.