The results of a small study prompted the researchers to advise people who have high blood pressure or heart disease to avoid energy drinks because they could impact their blood pressure or change the effectiveness of their medications.

The drinks generally have high levels of caffeine and taurine, an amino acid found in protein-rich foods like meat and fish that can affect heart function and blood pressure, the researchers say. They were unable to test the effect of higher levels of alcohol consumption, although drunkenness probably negates any positive effects on the brain.

"We saw increases in both blood pressure and heart rate in healthy volunteers who were just sitting in a chair watching movies. They weren't exercising. They were in a resting state," - says James Kalus of Henry Ford Hospital in Detroit, who led the study.

The increases did not rise to dangerous levels in the group of 15 healthy volunteers, whose average age was 26, the researchers say. But the increases potentially could be significant in people with cardiovascular disease or those taking drugs to lower heart rate or blood pressure.

The American Beverage Association industry trade group defends the findings.

"While the amount of caffeine in energy drinks or coffee may cause a slight and temporary increase in blood pressure, it would have no greater effect than walking up a flight of steps," it says in response to the study.

"So singling out energy drinks in a unique manner, particularly when compared to a more commonly consumed caffeinated beverage like coffee, does not provide a full and proper context for consumers."

Kalus declined to say which brand of energy drink was used in the study. He says the drinks generally contain similar ingredients.

"By giving the brand, it would dilute the message that all of these drinks need to be looked at," - he says.