caffeine
n. a CNS stimulant found in coffee, tea, cola, cocoa, chocolate, and certain prescribed and over-the-counter medications. It is an antagonist of the neuromodulator adenosine. Caffeine belongs to the methylxanthine group of alkaloids, and its effects include rapid breathing, increased pulse rate and blood pressure, and diminished fatigue. Precise effects vary with the amount ingested and the tolerance of the individual. Moderate doses produce an improved flow of thought and clearness of ideas, together with increased respiratory and vasomotor activity; large doses may cause caffeine intoxication and substance-induced anxiety disorder. Because of its stimulant effects, caffeine is used in wake-promoting agents and in certain analgesics and cold remedies containing ingredients that usually cause drowsiness. See also aspirin combinations.