Recent research from both Australia and the United States has raised questions about energy drinks and says they may be harmful to some people.
The researchers say high-caffeine energy drinks may provide more than an energy boost as they may also boost heart rates and blood pressure levels and increase the danger of blood clots.
The results of a small study by researchers from Detroit's Henry Ford Hospital has found that the high levels of caffeine and taurine, an amino acid found in protein-rich foods like meat and fish, can affect heart function and blood pressure.
They advise people who have high blood pressure or heart disease to avoid energy drinks because they could have an impact on their blood pressure or change the effectiveness of their medications.
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.
They say 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.
Energy drinks include 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 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.
Researchers from the Royal Adelaide Hospital are warning 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.
Red Bull which was first launched in Austria in 1987 is so-called after the ingredient taurine, the organic acid which was first isolated in a bull; it is prohibited as a soft drink in Norway, Uruguay, Denmark and Iceland.
The formula contains the same amount of caffeine as a cup of filter coffee, around 80mg and has benefited from much pop and sport celebrity endorsement.
The sugar-free version of the drink was launched in 2003.
Sales of Red Bull last year reached 3.5 billion cans, sold in 143 countries.