Sources: Various manufacturers. The Food Safety Promotion Board, Ireland. “A Review of the Health Effects of Stimulant Drinks,” final report. The Drug Info Clearinghouse of the Australian Drug Foundation.

Red Bull - 80mg of caffeine. 1,000mg of taurine. 600 mg glucuronolactone.

Rockstar - 75mg of caffeine. 946mg of taurine. 200mg of guarana. (all per serving - there are two servings in each can)

SoBe Adrenaline Rush - 79mg of caffeine. 1,000mg of taurine. 50mg of guarana.

Monster - 2,500mg "energy blend" of guarana, glucuronolactone, and 80mg of caffeine. 1,000mg of taurine. (all per serving - there are two servings in each can)

Starbucks doubleshot - 121mg of caffeine.

Coca-Cola - 23mg of caffeine per 8 fluid ounces. 34.5mg of caffeine per can.

A cup of coffee - In one 8 ounce cup of Starbucks coffee there are 200mg of caffeine. In one shot of espresso there are 89mg of caffeine.

Guarana: A plant native to South America, contains ingredients chemically similar to caffeine that act in the body much as caffeine does. Doctors and researchers say because guarana and caffeine are so similar, it is necessary to include both when estimating the total caffeine in a drink.

Glucuronolactone: Occurs naturally in the body and some people believe it provides an immediate kick of energy and purges the body of harmful substances. According to the Irish Food Safety Promotion Board, which made a study of energy drinks, there has been little research about the health effects of high consumption of glucuronolactone.

Taurine: Occurs naturally in the body, some people believe it detoxifies the body and purges harmful substances. Some consumers seek out drinks with taurine to replenish small amounts lost from increased stress or physical activity. There's been little research into its benefits or side effects.

Doctors say the high content of caffeine and similar ingredients in energy drinks causes dehydration, and that in addition to large quantities of sugar, the drinks are potentially dangerous. The four "energy" drinks below (Red Bull, Rockstar, SoBe Adrenaline Rush, and Monster) are among the best selling and most popular. They contain ingredients that other beverages with caffeine such as colas and coffee drinks do not.