Caffeine, or THEINE, , is the alkaloid or active principle of Coffee (q.v.) and Tea (q.v.). When isolated, it forms beautiful white crystals, with a silky lustre, which are soluble in chloroform, water, alcohol, and ether. It is a remarkable fact that nearly all the beverages in use in different parts of the world owe their virtues to, or at least contain, the same or similar active principles. Thus caffeine is present in coffee, tea, guarana, Paraguay tea or maté, and in the kola nut; while in cocoa theobromine is the corresponding principle, differing but slightly from caffeine in chemical composition—the latter being methyl-theobromine—i.e. theobromine in which one atom of hydrogen has been replaced by the group methyl, . In coffee, caffeine is present to the extent of from 1 to 2 per cent., while ordinary tea and guarana contain from 2½ to 6 per cent. In large doses, caffeine proves fatal to the lower animals, causing convulsions and death. In man, doses of 8 to 12 grains produce diuresis, great excitement, anxiety, and even delirium; but it has been of late used medicinally as a powerful stimulant of the heart's action. It may be extracted from coffee or tea by making a decoction in hot water, and adding acetate of lead, which causes a precipitate of caffeotannate of lead. When the latter is acted on by sulphuretted hydrogen, the lead is separated, and the caffeine left in solution. On evaporation of the liquid, and recrystallisation from alcohol, the caffeine separates in crystals.
Caffeine forms a series of salts, of which the citrate has come largely into use. One grain given every hour is often found to give great relief in sick headaches.