Energy drinks are beverages like Red Bull, Rock Star and Monster, which contain large doses of caffeine and other legal stimulants like guarana and ginseng. The amount of caffeine in an energy drink can range from 75 milligrams to over 200 milligrams per serving. This compares to 34 milligrams in Coke and 55 milligrams in Mountain Dew. For more information on caffeine content of energy drinks and other products, click here.
If a drink advertises no caffeine, the energy comes from guarana, which is the equivalent of caffeine. 5-hour energy drink advertises "no crash," but this claim is referring to no "sugar crash" because the drink has artificial sweetners.
Any vitamins or amino acids like taurine are better found by eating a variety of foods and taking a daily vitamin and mineral supplement.