Xn) RPhrases = R22 SPhrases = S2 NFPA-H = 2 NFPA-F = 0 NFPA-R = 0 LD50 = 192 mg/kg (rat, oral)

Caffeine is a bitter, white crystalline xanthine alkaloid that is a psychoactive stimulant drug. Caffeine was discovered by a German chemist, Friedrich Ferdinand Runge, in 1819. He coined the term kaffein, a chemical compound in coffee (the German word for which is Kaffee), which in English became caffeine (and changed to Koffein in German).

Caffeine is found in varying quantities in the beans, leaves (leaf), and fruit of some plants, where it acts as a natural pesticide that paralyzes and kills certain insects feeding on the plants. It is most commonly consumed by humans in infusions extracted from the bean (coffee bean) of the coffee plant (Coffea arabica) and the leaves of the tea bush (Camellia sinensis), as well as from various foods and drinks containing products derived from the kola nut. Other sources include yerba mate, guarana berries, and the Yaupon Holly.

In humans, caffeine is a central nervous system (CNS) stimulant, having the effect of temporarily warding off drowsiness and restoring alertness. Beverages containing caffeine, such as coffee, tea, soft drinks, and energy drinks, enjoy great popularity. Caffeine is the worlds most widely consumed psychoactive substance, but, unlike many other psychoactive substances, it is legal and unregulated in nearly all jurisdictions. In North America, 90% of adults consume caffeine daily. The U.S. Food and Drug Administration (Food and Drug Administration (United States)) (FDA) lists caffeine as a "multiple purpose generally recognized as safe food substance".

Caffeine has diuretic properties when administered in sufficient doses to subjects that do not have a tolerance (drug tolerance) for it. Regular users, however, develop a strong tolerance to this effect, and studies have generally failed to support the common notion that ordinary consumption of caffeinated beverages contributes significantly to dehydration.