Caffeine, also called trimethylxanthine (C8H10N4O2), a white, bitter alkaloid extracted from coffee, and also found in tea, cocoa, cola, and ilex (holly) plants. Caffeine stimulates the central nervous system and heart, and is a diuretic. It increases alertness, in excess causing insomnia, and is mildly addictive. It is often included with aspirin or codeine in analgesic preparations. Melting point 238°C.