The BBC's Yigal Chazan: "Prescribed drugs can have more effect than alcohol"

In a month-long project, researchers at Surrey University assessed the road handling skills of 16 volunteers who had been given some of the most commonly prescribed anti-depressants.

They found the medicines delayed reaction times twice as much as the legal limit of alcohol.

The research is the latest in a body of evidence suggesting that drivers taking anti-depressants and tranquillisers can pose a serious threat to road safety.

The Surrey University volunteers were asked to hit the brake of the car in which they were sitting as quickly as possible, every time a red lamp mounted on the car in front was switched on.

They found that those volunteers who had taken tricyclic anti-depressants delayed reaction times by 120 milliseconds.

Professor Ian Hindmarch, head of human psychopharmacology, said at motorway speeds the delay would significantly increase the risk of a road traffic accident.

He said: "At 70mph, with the anti-depressants your car would actually travel a whole car's length - 12 feet - before your foot hit the brake pedal, whereas if you were illegally driving your car at blood alcohol concentration of 80mg percent you would travel five or six feet."

Vigilance impaired

The tests corroborate previous studies showing that anti-depressants cause drowsiness and blurred vision and can seriously affect vigilance and anticipation.

Other studies have looked at the prescription records of crash victims and found that drivers taking these drugs have a much greater risk of having an accident.

One study by Dundee University concluded there would be 1,600 fewer car crashes and 110 fewer deaths in Britain each year, if people on tranquillisers refrained from driving.

Professor Tom MacDonald, an expert in clinical pharmacology and one of the authors of the Dundee research, said: "One in 10 people gets a prescription for a benzodiazipine.

"My opinion is that all doctors should now tell their patients they should not drive if they take one of these angiolytic drugs."

The Royal Automobile Club is concerned at the growing body of evidence that prescribed drugs are a danger to drivers.

Road safety manager Kevin Delaney said: "They slow down the rate at which you are able to perceive things, they may affect your ability to perceive hazards and just as importantly they impair your reaction time in a way that is very similar to quite significant amounts of alcohol."

The RAC is calling for the UK to adopt the "traffic light" system of labelling for drugs favoured by some European countries, with red, amber and green indicating the level of risk of each medication.

Mr Delaney said: "People's perception is that prescribed drugs are not harmful.

"After alll, people take prescribed drugs to make them better, they do not think of them as being harmful in the same way that they recognise alcohol and driving as harmful."

Roadside screening

Last year the government conducted trials of roadside screening devices for both prescribed and illicit drugs following a Department of Transport study showing that a fifth of accident victims had traces of illegal substances in their bodies.

It was found, however, that the figure for legal medicines was much smaller.

Some cite this as evidence that the level of risk they pose is unclear.

Dr James Bevan, senior medical consultant for the Automobile Association, says: "Initially when you take a tricyclic it will slow down your reaction time and your driving ability.

"However, hopefully in two or three weeks as the depression lifts you will be safer on the roads, and also the sedative side-effects of the tricyclic drugs become less and you are a safer driver."

A government spokesman said doctors and pharmacists should make patients fully aware of the possible risk of taking prescribed drugs.

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