One in six drivers admits that they could be driving impaired, not because of alcohol or cannabis but because they have been taking over-the-counter or prescription drugs, many of which come without any warnings, according to a new survey.

Doctors say that such medication, which includes easily bought hay fever remedies, can make users so sleepy that they increase the risk of a crash occurring by almost sixfold.

The study, carried out by road safety charity Brake and motoring assistant Green Flag, shows that anti-hystemines, anti-depressants and anti-anxiety pills all put drivers at risk.

The chief executive of Brake, Mary Williams, said: 'The Government must do more to make drivers aware that their medication may make them unsafe behind the wheel. There needs to be clear warnings on all prescription and over-the-counter medication.' The warning comes after researchers found that a normal dose of these tablets can easily impair driving to a greater extent than the legal alcohol limit.

Professor Ian Hindmarch, head of the medical research centre at Surrey University, said: 'The vast majority of anti-hystemines are sedating and some have exactly the same ingredients as many sleeping tablets. Yet many of these are available over the counter without warning labels.'

A study looking at fatal accidents where motorists had used alcohol, cannabis or anti-hystemines gave worrying results. In normal circumstances, the cause of an accident would be a variety of factors, but where anti-hystemines had been used 72 per cent of these drivers were to blame for the crash, and with anti-anxiety tablets it was 98 per cent, higher than in the case where the driver was drunk, which was 87 per cent.

Hindmarch added: 'If the drugs are prescription, the pharmacist will say, don't drive if you feel drowsy, but even so people don't realise when their driving will be impaired. People say they feel great after a couple of drinks but they are not capable of driving. And lots of this medication is bought without any medical advice, straight over the counter.'