Getting behind the wheel while hungover can be just as dangerous as driving after too many beers, research suggests.
Researchers say drivers should be warned that the effects of heavy drinking extend beyond the point when traces of alcohol have disappeared from the blood.
A Dutch study, being presented at a conference in Brisbane this week, put volunteers through simulated driving tests the night after they consumed about 10 alcoholic drinks.
They took the tests after their blood alcohol concentrations had returned to zero, and the results were compared with how they performed after a night when they did not drink.
The study shows hangovers can significantly increase the number of lapses in attention, and weaving on the road.
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"The magnitude of driving impairment is higher than that observed with a blood alcohol concentration of 0.05 per cent, which is the legal limit for driving in many countries," says Dr Joris Verster of Utrecht University.
A smaller UK study produced similar findings.
It shows hungover drivers are more likely to travel at an inconsistent speed, make more driving errors and weave on the road. They also have slower reaction times.
Professor Chris Alford of the University of the West of England says the effect of hangovers on driving performance must be incorporated into driver safety campaigns.
Other research has shown some people's claim to be immune from developing hangovers is a myth.
A Canadian study of almost 800 university students suggests most drinkers are susceptible, and the impact is largely related to how intoxicated they were.
The hangover study is being presented at the Australasian Professional Society on Alcohol and other Drugs conference in Brisbane.
