Researchers at Johns Hopkins University in Baltimore, Md., found that some energy drinks are so packed with caffeine they should carry prominent warning labels.

The researchers say some drinks have up to 14 times the caffeine of a regular can of soda – which is the equivalent of downing 14 cans of coca-cola or seven cups of extra-strong coffee.

"The caffeine content of energy drinks varies, yet the amounts are often unlabelled and few include warnings about he potential health risks,” Dr. Roland Griffiths wrote in the journal Drug and Alcohol Dependence.

According to the National Institutes of Health, those risks include rapid heart rate, nausea, vomiting, restlessness, anxiety, depression, tremors, and difficulty sleeping.

The NIH defines moderate caffeine intake as three 8 oz. cups of coffee per day and excessive intake as ten 8 oz. cups of coffee per day.

Because many energy drinks are marketed as "dietary supplements," the limit that the Food and Drug Administration requires on the caffeine content of soft drinks (71 milligrams per 12-ounce can) does not apply. The caffeine content of energy drinks varies from 50 to more than 500 milligrams.

"It's notable that over-the-counter caffeine-containing products require warning labels, yet energy drinks do not," Chad Reissig, one of the study's authors, said in a news release.

"It's like drinking a serving of an alcoholic beverage and not knowing if its beer or scotch," Griffiths said.

Researchers are concerned these energy drinks might serve as “gateway” products to more serious abuse of drugs.