Caffeine, an alkaloid (C₈H₁₀O₂N₄·H₂O) found in coffee, tea, cacao, and some other plants. It is also present in most cola beverages. Caffeine was discovered in coffee in 1820. In 1838 it was established that theine, discovered in tea in 1827, is identical to caffeine. The drug increases the blood pressure, stimulates the central nervous system, promotes urine formation, and stimulates the action of the heart and lungs. Caffeine is used in treating migraine because it constricts the dilated blood vessels and thereby reduces the pain. It also increases the potency of analgesics such as aspirin, and it can somewhat relieve asthma attacks by widening the bronchial airways. Caffeine is produced commercially chiefly as a byproduct in making caffeine-free coffee (see Coffee).

Caffeine has been suggested as a possible cause of cancer and of birth defects. No studies, however, have yet confirmed any of these charges. Persons who stop drinking coffee do sometimes experience withdrawal headaches.