Energy drinks such as Red Bull, Monster, Amp and others need warning labels, a caffeine researcher says.

Survey: Warning Labels On Energy Drinks?

"The caffeine content of energy drinks varies over a 10-fold range, with some containing the equivalent of 14 cans of Coca-Cola, yet the caffeine amounts are often unlabeled and few include warnings about the potential health risks of caffeine intoxication," said Roland Griffiths, of the Johns Hopkins Medical Institutions.

Caffeine intoxication causes nervousness, anxiety, restlessness, insomnia, gastrointestinal upset, tremors, rapid heartbeats and restlessness, according to a news release. In rare cases, it can cause death.

He said the uncertainty about how much caffeine a drink contains would be like having an alcoholic drink and not knowing if it was beer or scotch.

The news release said people in the U.S. spend about $5.4 billion on energy drinks anually. In a 2007 survey of 496 college students, 51 percent reported consuming at least one energy drink during the last month; 19 percent reported heart palpitations from drinking energy drinks.

A regular 12-ounce cola drink has about 35 milligrams of caffeine, and a 6-ounce cup of brewed coffee has 80 to 150 milligrams of caffeine. 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," said Chad Reissig, Griffiths' co-author. Their work appears in the journal Drug and Alcohol Dependence.