Doctors researching the effects of caffeine are calling for warning labels on popular energy drinks, some of which contain caffeine levels equivalent to 14 cans of Coca-Cola.

Experts are concerned the labels do not make clear just how much caffeine is in these drinks, leading consumers to take unnecessary health risks.

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The market for energy drinks has exploded in the United States since the introduction of Red Bull in 1997, with $5.4 billion in sales. The market is growing 55 percent a year, according to the Johns Hopkins University researchers, who conducted the study.

The active ingredient in most drinks is caffeine. A 12 oz. can of Coke contains 35 milligrams of caffeine, a 6 oz cup of coffee contains about 100 mg, and a 8.3 oz can of Red Bull contains 80 mg. But researchers warn some energy drinks can contain more than 500 milligrams of caffeine.

Caffeine use has been associated with headaches, nausea and insomnia in moderate amounts. Large doses of caffeine can trigger rapid heartbeat, tremors, and in rare instances, death, researchers found.

The FDA has approved a caffeine level of 71 milligrams per can for soda, but most energy drinks are listed as dietary supplements, which do not have FDA caffeine restrictions.

Researchers point out that the FDA requires a warning on "No Doz" pills, which contain 100 milligrams of caffeine. But there is no similar label for energy drinks which have caffeine levels double, triple or even quintuple as much.

The study's authors also worry that high-caffeine energy drinks could be a gateway drug for controlled substances, including amphetamines and Ritalin.