Prescribed medicines and the UK drug-driving law

Since the drug-driving offence was introduced in England and Wales, a set of named drugs has a specified blood level above which driving is an offence. The THINK! road-safety campaign describes this as applying to 17 controlled drugs, covering both illegal substances and some medicines. Scotland operates a similar approach. On top of this sits the older offence of driving while unfit through drugs, which applies to any substance, whether or not it appears on a list.

That second point matters for people who take medicines that are not on the named list at all. A strong antihistamine, a sedating antidepressant or a night-time cold remedy can each leave a driver drowsy. If that impairs driving, the person can still be committing an offence and, more importantly, can cause a collision.

The prescription medicines named on GOV.UK are amphetamine and related drugs (including dexamphetamine and selegiline), the benzodiazepines clonazepam, diazepam, flunitrazepam, lorazepam, oxazepam and temazepam, methadone, and morphine and opioid-based drugs including codeine, tramadol and fentanyl. Most people on these are being treated for anxiety, sleep problems, epilepsy, pain, attention disorders or opioid dependence.

Being on the list does not mean you cannot drive. It means the blood level is policed, and that the defence described below is what protects a patient who is taking the medicine properly.

GOV.UK explains that you may legally drive if you were prescribed the medicine and followed the advice of a healthcare professional on how to take it, provided it is not impairing your driving, even if your blood level is over the specified limit. The Department for Transport guidance written for clinicians describes the same defence and says patients should normally continue medicines as advised by their doctor or by the patient information leaflet.

Practical consequences follow from the wording. Taking more than prescribed, mixing the medicine with alcohol or with someone else's tablets, or driving when you feel sedated all weaken the position. The defence is not a licence to drive impaired.

• Keep medicines in their labelled pharmacy packaging so the prescriber, dose and your name are visible.

• Keep a note of when you started a medicine or changed dose; the first days and any dose increase are the riskiest.

• Ask the prescriber or pharmacist, in your own words, whether this particular medicine is likely to affect driving for you.

• Do not stop a medicine abruptly to be safe to drive; speak to the prescriber first.

Medicine leaflets and labels often carry a warning about drowsiness. Treat the following as reasons to stay off the road until you have spoken to a clinician: feeling sleepy during the day, blurred or double vision, dizziness on standing, slowed reactions, difficulty concentrating, or falling asleep without warning.

The NHS page for co-beneldopa, a Parkinson's medicine, is a useful illustration: it says some people feel dizzy or very sleepy or fall asleep suddenly, even in the day, and that anyone affected should not drive, cycle or use tools or machinery. By contrast, the NHS page for azathioprine says the medicine does not usually impair driving, although you should not drive if symptoms affect your concentration. The same question gets different answers for different drugs, so check each one.

Police can carry out a field impairment assessment if they suspect impairment, and roadside screening is available for cannabis and cocaine. If screening or an assessment suggests drugs, blood or urine testing is done at the station to confirm what is present.

Telling the officer that you take a prescribed medicine, and showing the packaging, is sensible. Whether the defence applies is later decided on the facts: that the medicine was prescribed, taken as advised and not impairing you.

The best time to ask about driving is at the start of treatment. A short conversation can cover how soon the drug takes effect, whether drowsiness usually wears off after the first week, whether the evening dose is better, and how alcohol interacts. People whose job depends on driving should say so, because the answer may change which medicine is chosen.

Some conditions have their own driving rules enforced by the DVLA, and those are separate from the drug-driving law. Existing guides on this site cover diabetes and driving and seizures and driving.