It may come as no surprise that under new regulations which came into force last month drivers face prosecution if they have taken illegal drugs such as cannabis and cocaine.
But the new rules also include eight common legally prescribed drugs such as oxazepam and diazepam which are used to relieve anxiety and temazepam which is prescribed for insomnia.
To defend themselves against possible prosecution drivers need to ensure that they have evidence that they are taking prescription drugs legally, advises Just Motor Law specialist Matt Reynolds.
“This law potentially poses a huge issue to those driving while taking properly prescribed medication,” he warns.
“Everybody metabolises and excretes drugs at a different rate, meaning some people may be more likely to have residual levels of drugs in their system than others, despite taking the same dose.
“However the good news is that those who face prosecution because of the levels of prescription drugs in their system may be able to raise a medical defence if they are taking the medicine in accordance with the advice of their doctor and the patient information leaflets provided by the manufacturer.
“I recommend patients visit their doctor in order to obtain a letter confirming their legitimate use of the drug they have been prescribed and to carefully read the advice leaflets that accompany their medication.
“It is in no way a guarantee that they will avoid prosecution but it’s a practical and beneficial first step to a medical defence.”
Although only recently introduced, the new law is already stirring up controversy.
Greater Manchester Police announced it would delay implementing the new rules until it had examined the legal implications of the changes.
But Daily Express GP Dr Rosemary Leonard says: “If you are taking your medication according to your doctor’s advice you are unlikely to run into any trouble.
“However if you are stopped because your driving is giving cause for concern and you are found to have prescription drugs affecting your driving, then you could be prosecuted.”
The penalties for drug-driving are on a par with those for drink-driving.
Those who are found guilty will face a minimum 12-month driving ban, a criminal record and a fine of up to £5,000 or up to six months in prison.