"The caffeine content of energy drinks varies over a ten-fold range, with some containing the equivalent of 14 cans of Coca-Cola -- yet the caffeine amounts are often unlabelled and few include warnings about the potential health risks of caffeine intoxication," said Dr Roland Griffiths of Johns Hopkins University, co-author of the study.

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

As energy drinks have grown in popularity, hospitals have seen an attendant rise in admissions from patients who have consumed these products -- often with alcohol.

"Alcohol adds another level of danger because caffeine in high doses can give users a false sense of alertness that provides incentive to drive a car or in other ways put themselves in danger," Dr Griffiths said.

Caffeine has a number of undesirable side-effects such as anxiety, restlessness, insomnia, gastrointestinal distress, tremors, rapid heartbeat, and in rare cases, death.

Yet there is a currently a disparity between labelling requirements for medicines containing caffeine and energy drinks.

"It's notable that over-the-counter caffeine-containing products require warning labels, yet energy drinks do not," says Dr Chad Reissig, Ph.D., another co-author of the study.

But the drinks industry has yet to face serious pressure to change the way that these products are labelled.

Their position is bland and non-committal.

"All energy drinks should be consumed in moderation," said a spokesperson for the British Soft Drinks Association.