Caffeine is substance that exists naturally in plants. It can also be produced synthetically and used as an additive in certain food products. It is a central nervous system stimulant and a diuretic.

Caffeine is absorbed and distributed very quickly. After absorption, it passes into the central nervous system, or the brain. "Caffeine sensitivity" refers to the amount of caffeine that will produce negative side effects. This amount will vary from person to person. Caffeine does not accumulate in the bloodstream nor is it stored in the body. It is excreted in the urine many hours after it has been consumed. Caffeine will not reduce the effects of alcohol, although many people still believe a cup of coffee will "sober up" an intoxicated person.

Caffeine is widely consumed. It is found naturally in the leaves, seeds, and fruits of more than 60 plants, including tea leaves, kola nuts, coffee, and cocoa beans. It is in coffee, tea, chocolate, cocoa and many carbonated beverages such as colas.