Motorists taking prescription drugs may face prosecution if proposed changes to the road laws are adopted by the government.
The new rules are intended to deter users of illegal substances from driving, but they may also affect drivers who have taken medication for legitimate reasons.
The recommendation that police be equipped with road side testing equipment for drug use is expected to be made in a report by Sir Peter North. The same document is believed to advise ministers that the drink-drive limit should be reduced from 80mg of alcohol per 100ml of blood to 50mg.
At the moment motorists suspected of being under the influence of drugs are subjected to a roadside test of their ability to drive. This method is still used in the US to measure sobriety and includes walking in a straight line and counting backwards from ten.
The police have experimented with a number of electronic devices which detect drugs, including those based on an analysis of sweat or salvia, but none have been adopted nationwide.
It is thought that the report, due to be published this week, will advise the Home Office to provide police officers with the means to verify drug use at the roadside, in the same way they are currently use breathalysers for alcohol.
However, while the consumption of certain medications are liable to affect a users' ability to drive, any drug-driving law designed to cover legitimate use would require a legal limit. Contributors to the review admit that this would be problematic to set up.
Dr Vivienne Nathanson, head of science and ethics at the British Medical Association, told the Sunday Times: "At the moment we do not know enough about how drugs affect driving to be able to set limits."
She added: "We need to identify the drugs most commonly found in accidents, including therapeutic drugs, then carry out research. Once we have a base we are targeting, we would then be in favour of testing for drugs."