Energy drinks usually contain a combination of caffeine and other ingredients like herbs or vitamins. Common ingredients include B vitamins, taurine, ginseng and carnitine. Guarana — an herb that contains large amounts of caffeine — may be added to energy drinks in addition to synthetic caffeine.
According to the University of California Davis, some energy drinks contain several servings per can, resulting in total caffeine doses as high as 294 milligrams. Many of these products also contain large amounts of sugar, corn syrup or artificial sweeteners.