Caffeine is a stimulant of the methylxanthine class, and is the most commonly consumed psychoactive drug globally; it is nearly universally legal to produce and sell, usually in the form of caffeinated drinks. It is mainly used for its eugeroic, ergogenic, or nootropic (cognitive-enhancing) properties ; it is also widely used recreationally, often in social settings. Caffeine acts by blocking the binding of adenosine at a number of adenosine receptor types, inhibiting the centrally depressant effects of adenosine and enhancing the release of acetylcholine. Caffeine has a three-dimensional structure similar to that of adenosine, which allows it to bind and block its receptors. Caffeine also increases cyclic AMP levels through nonselective inhibition of phosphodiesterase, increases calcium release from intracellular stores, and antagonizes GABA receptors, although these mechanisms typically occur at concentrations beyond usual human consumption.