The US Food and Drug Administration is unimpressed by the fad for drinks that contain a double hit - alcohol and caffeine. Unless makers supply the FDA with scientific evidence that the drinks are safe they could be banned within months.

The agency is worried that consuming the drinks - which can mask the effect of alcohol - leads to rash behaviour, car crashes, violence and assaults. The FDA issued the ultimatum last week in response to a request made by the National Association of Attorneys General. "Caffeine added to alcohol poses a significant public health threat," said a task force headed by the attorney-generals of Utah, Guam and Connecticut.

The FDA allows caffeine concentrations of up to 200 parts per million in soft drinks, but adding caffeine to alcohol is unregulated. At least two of the 27 companies contacted have already withdrawn their drinks.

In 2006, Cecile Marczinski and Mark Fillmore of the University of Kentucky found that consumers of the drinks felt they were less inebriated than when imbibing alcohol alone, even though they made just as many errors in standard tests of alertness and reaction time.