The most commonly consumed psychoactive substance globally. It is mainly used for its eugeroic, ergogenic, and nootropic properties. It is also used recreationally or in social settings.

Caffeine is a naturally occurring stimulant found in coffee beans, tea leaves, cacao, and several other plants. It works primarily by blocking adenosine receptors in the brain, reducing feelings of tiredness and increasing alertness. Caffeine can also temporarily improve attention, reaction time, and physical performance. Its effects usually begin within minutes and may persist for several hours, depending on dose and individual metabolism. In larger amounts, it can cause restlessness, insomnia, increased heart rate, or anxiety. It is used in beverages, foods, medications, sports products, and laboratory research.