Dr. James Kalus who led the study, says increases were seen in both blood pressure and heart rate in 15 healthy volunteers with an average age of 26, who were resting and not exercising and while the increases did not rise to dangerous levels in the group they could be significant in people with cardiovascular disease or those taking drugs to lower heart rate or blood pressure.

The energy drinks included Red Bull, Full Throttle, Amp and Rush.

For the study the group were asked not to consume other forms of caffeine for two days before and throughout the study in which they consumed two cans of energy drinks daily over seven days, each can containing 80 milligrams of caffeine and 1,000 milligrams of taurine.

The volunteers' heart rates rose by about 8 percent on the first day and 11 percent on the seventh day - systolic blood pressure rose by 8 percent on the first day and 10 percent on the seventh day, and diastolic blood pressure rose by 7 percent on the first day and 8 percent on the seventh day.

The researchers suggest the caffeine and taurine in the drinks were responsible for the changes.

Australian researchers from the Royal Adelaide Hospital say they have found that the sugar-free version of Red Bull may increase the danger of blood clots; they say it creates "sticky" blood, raising the risk of heart attack or stroke and warn that the drink "could be deadly" for people with heart abnormalities.

Lead researcher Dr. Scott Willoughby says though the incidence of sudden cardiac death is very low, the drink could be more deadly for people who have an unknown cardiovascular abnormality.

Sales of Red Bull last year reached 3.5 billion cans, sold in 143 countries.