Experts have challenged UK government's advice that urges drivers to drink caffeinated drinks if they feel tired, saying that the stimulant could have potentially dangerous side-effects.
A motoring group and a car parts store chain said drivers who rely on energy drinks and coffee to keep them awake are at a higher risk of both increased aggression and worse fatigue when the caffeine's effects are over.
Halfords and the Institute of Advanced Motorists also said excessive consumption of energy drinks could produce symptoms similar to alcohol intoxication.
Meanwhile, sleep experts described the claims as "fear-mongering" and said research showing the beneficial effects of combining caffeine with naps had led to it being added to the Highway Code.
Halfords expressed concern about the lack of information about the effects of caffeine drinks and how much drivers should consume.
The firm said other studies showed that one hour after drinking a highly caffeinated and sugared drink, some motorists suffered delays in their reaction times.
"Caffeine drinks and coffee are often recommended as an antidote to tiredness, but the side-effects are less well promoted," the Scotsman quoted Mark Dolphin, Halfords motoring expert, as saying.
"The best advice for drivers is to make sure they have a good night's rest before setting out on a long journey. They should stop at least every two hours, and if feeling tired, should stop and have a break," said Dolphin.
Neil Greig, of the Institute of Advanced Motorists, said: "We would like to see small warnings on cans informing people that the effects can be short-lived and not to consume too much of these products.
"Energy drinks are good as a quick fix, but they're no substitute for regular breaks.
"Having a high-caffeine drink is a one-off hit - you can't repeat it, as this type of drink does not produce the same effect in a couple of hours' time," said Greig.