A study by researchers in the U.S. has found that for some people high energy drinks may be dangerous.
The researchers from Wayne State University say energy drinks contain chemicals that may be harmful to people with heart conditions.
Though energy drinks may help a person to exercise they also raise heart rates and blood pressure; in contrast, "sports drinks" usually contain various mixtures of water, sugars and salts alone, without chemicals aimed at increasing "energy" or alertness.
Energy drinks contain chemicals such as caffeine and taurine and while they make people more alert they also stimulate the heart and raise blood pressure.
The researchers suggest they may even be harmful to consumers who are not exercising but simply sitting still.
According to Dr. James Kalus from the Henry Ford Hospital in Detroit who led the study, increases in both blood pressure and heart rate were seen in healthy volunteers with an average age of 26, who were just watching movies and were in a resting state.
Although the change seen did not rise to dangerous levels among the volunteers, the researchers say it could be significant in people with cardiovascular disease or those taking drugs to lower heart rate or blood pressure.
The 15 volunteers each drank two cans of energy drinks daily over a seven day period with each can containing 80 milligrams of caffeine and 1,000 milligrams of taurine.
They were asked to abstain from other forms of caffeine for two days prior to and throughout the study.
On the first day their heart rates rose by about 8 per cent but rose by 11 per cent on the seventh day.
The researchers say while energy drinks increase concentration and wakefulness, people with risk factors for heart disease could have a bad reaction.
Kalus advises those with high blood pressure and heart disease to avoid such drinks because they could affect their blood pressure and may even alter the effectiveness of their medications.
Experts say the study raises some concerns as energy drinks could affect some individuals unaware that they had a problem.
The study was presented at the American Heart Association's Scientific Sessions 2007.