text: what is caffeine?? description of drug caffeine is a xanthine alkaloid found in the leaves and beans of the coffee tree, in tea, yerba mate, guarana berries, and in small quantities in cocoa, the kola nut and the yaupon holly. in plants, caffeine acts as a natural pesticide that paralyzes and kills many insects feeding upon them.  caffeine is a central nervous system (cns) stimulant, having the effect of warding off drowsiness and restoring alertness.in nature, caffeine is found with widely varying concentrations of the other xanthine alkaloids theophylline and theobromine, which are cardiac stimulants. when caffeine appears to have different effects depending on the source, it is due primarily to varying concentrations of other stimulants and absorption rates of the mixture.