DRIVERS taking prescription tranquillisers were responsible for four out of five collisions in which they were injured, new research has shown.

Researchers led by Edward Ogden, of Swinburne University, made the finding after analysing blood samples from 1800 drivers treated in Victorian hospitals after a car crash over 15 months between 2009 and last year.

In a separate analysis, they used police records to assign injured drivers to three groups - those responsible for their collision, those who contributed to it and those who were not responsible.

The researchers found that a high percentage of drivers affected by illicit drugs were responsible for their collision, including 88 per cent affected by stimulants including amphetamines and 83 per cent affected by cannabis.

But benzodiazepines - prescription tranquillisers commonly used to treat anxiety and insomnia - also emerged as a significant factor in collisions, particularly when combined with alcohol.

Dr Ogden said 184 drivers were affected by benzodiazepines, and 84 per cent were responsible for their collision.

''When you look at people with very high or toxic levels of benzodiazepines, or benzodiazepines plus alcohol, 100 per cent were responsible,'' he said.

More than 5 million benzodiazepine prescriptions are subsidised each year through the Pharmaceutical Benefits Scheme.

Of particular concern was the powerful and rapid-acting drug alprazolam (also known by the brand name Xanax), which drug and alcohol experts warn is being increasingly misused.

Dr Ogden said alprazolam was ''incredibly impairing'' in his study, which he will discuss at the Australasian Professional Society on Alcohol and other Drugs conference in Melbourne today.

Of the 44 drivers affected by alprazolam, 43 recorded high or toxic doses or combined it with alcohol.

''These drugs have all got legitimacy in treatment but it raises the question: should people taking minor tranquillisers be held to a zero blood alcohol limit?'' Dr Ogden said.

''Putting aside people who are buying them on the streets, people who are legitimately using benzodiazepines should be warned that the combination of minor tranquillisers and alcohol is a disaster.''

Dr Ogden said huge progress had been made in reducing the road toll through campaigns against drink driving, but drugs were the next frontier.

''We need to start a debate,'' he said.

''It's really easy to say it's only those other people who take illicit drugs - but prescribed medicines can be a problem and particularly those that are misused.''

Police conducted 40,000 roadside drug tests for speed, ecstasy and cannabis last financial year and issued 1500 fines for drug driving.

Dr Ogden's study found that 508 drivers were affected by alcohol, and 96 per cent were responsible for their collision. Of more than 900 drivers unaffected by alcohol or drugs, 45 per cent were responsible for their collision.