Some energy drinks contain as much caffeine as 14 cans of Coca-Cola and have no labels warning consumers about potential health risks, say researchers with Johns Hopkins University in Baltimore.

The scientists reviewed energy drinks and published their findings in the September issue of Drug and Alcohol Dependence.

Discovering that the caffeine content of such drinks varies from 50 to more than 500 milligrams, they argue for prominent labelling so consumers know whether they are getting a little or a lot of caffeine.

"It's like drinking a serving of an alcoholic beverage and not knowing if it's beer or scotch," study co-author and professor of behavioural biology Roland Griffiths said in a news release Wednesday.

Caffeine intoxication is marked by nervousness, anxiety, restlessness, insomnia, gastrointestinal upset, tremors, rapid heartbeats, agitation and in very rare cases, death.

While the researchers highlighted their concerns over potential for inadvertent caffeine intoxication, Griffiths also noted that most of the drinks advertise their products as performance enhancers and stimulants. He said that's a marketing strategy that may put young people at risk for abusing even stronger stimulants.

In Canada, caffeine does not have to be listed on labels unless it has been added to the product separately as a pure substance, says Health Canada.

While there are many brands marketed as energy drinks in Canada, only Red Bull Energy Drink is authorized for sale as a natural health product.

Health Canada recommends consumers limit themselves to 500 millilitres or two cans of Red Bull a day, as indicated on the product label. It also advises against mixing the energy drink with alcohol.

Health Canada says it has received four reports of adverse reactions involving energy drinks, with symptoms including electrolyte disturbances, nausea and vomiting, and heart irregularities.

For women of childbearing age, it recommends a maximum daily caffeine intake of no more than 300 milligrams, or a little over two eight-ounce (237 ml) cups of coffee.

For the rest of the general population of healthy adults, the long-standing advice still applies of no more than 400 mg of caffeine per day, the equivalent of about three eight-ounce (237ml) cups of brewed coffee.