European researchers have discovered that a hungover driver who has no alcohol in their blood can be more dangerous behind the wheel than someone with a blood alcohol reading of 0.05.

Associate Professor Chris Alford from the University of the West of England says his volunteers were asked to drink heavily in the name of science and then undergo simulated driving tests when sober the next day.

"Things like their speed of reaction was slowed down, their variability in the way they drive was more erratic when they had a hangover," he said.

"We also found that they're making errors things like crossing the central lines, the lane markings in the road.

"So you could say their driving was as if they were over the legal limit of alcohol but of course they didn't have that alcohol on board anymore."

In a separate study in Holland, Dutch volunteers were given 10 drinks and then tested behind the wheel for an hour the following day.

Dr Joris Verster from Utrecht University says the driver's blood alcohol was back to zero, but their hangover far from over.

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"Driving was much worse when they had a hangover when we compared it to our results from previous studies [of drunk drivers]," he said.

Dr Verster says the symptoms of a hangover vary between people and from occasion to occasion.

"What's most often experienced is reduced concentration and alertness. Those are the things that are especially important for driving," he said.

Professor Alford says there are ways to test drivers for the symptoms of hangovers.

"There are some biochemical tests that you can do because some of these things hang around in your body for longer even though the alcohol is gone," he said.

"You can see the residual effects in some of your body chemistry, which you could actually test for if you wanted to.

However Professor Alford recommends education is the best course of action to convince people to think twice about driving hungover.

"I think above and beyond that, punishing people through convicting them and things is one thing but surely a better thing is just to educate people and have campaigns to say 'look, think about it'," he said.

The research is being presented at the Australasian Professional Society on Alcohol and other Drugs conference in Brisbane this week.