This is the molecular structure of caffeine. Caffeine is a central nervous system stimulant. It is the most widely consumed psychoactive drug. Caffeine is legal and unregulated in nearly all parts of the world. Caffeine is that it reversibly blocks the action of adenosine on its receptors and consequently prevents the onset of drowsiness induced by adenosine. Caffeine has the chemical formula of C8H10N4O2.