Caffeine

Caffeine (1,3,7-trimethylxanthine) is the world's most widely consumed psychoactive compound and one of the most extensively studied ergogenic aids. Its primary mechanism is competitive antagonism of adenosine A1 and A2A receptors in the brain — adenosine being the endogenous sleep-promoting neuromodulator that accumulates during wakefulness. By blocking adenosine binding, caffeine increases firing rates of dopaminergic, noradrenergic, and cholinergic neurons, producing heightened alertness, improved reaction time, reduced perception of effort, and enhanced working memory. At athletic doses (3–6 mg/kg body weight), caffeine increases time to exhaustion by 12–15%, improves high-intensity sprint performance, and enhances muscular endurance via calcium sensitisation in contractile fibres.