What gives coffee its kick? Caffeine, of course. Caffeine is trimethylxanthine (C8H10N4O2). It's an addictive stimulant that operates in the brain the same way amphetamones, cocaine and heroin do (although caffeine is much milder than those drugs). Caffeine occurs naturally in a number of plants, including coffee beans. Your average 6-ounce cup of drip-brewed coffee contains 100 mg of caffeine.