Driving on a prescription: the law and the medical defence

You may be able to drive while prescribed medical cannabis, but only within strict limits, and never if you are impaired. Here is the law in plain terms, and where to get advice on your own situation.

The short answer

There is a legal limit for THC in your blood while driving, set very low. As a prescribed patient you have a statutory medical defence that can allow you to drive above that limit, but only if you meet its conditions, and it never covers driving while impaired. The safe rule is simple: if there is any doubt, do not drive.

The limit, and the defence

UK law sets a THC limit of 2 micrograms per litre of blood, and it is an offence to drive above it. Because that limit is so low, most patients taking THC would exceed it. The law accounts for this with a statutory medical defence under section 5A of the Road Traffic Act 1988. It can apply if all three of these are true: your medicine is lawfully prescribed, you are taking it in line with your specialist’s instructions, and you are not impaired. Whether the defence applies in any given case is ultimately for the courts.

Impairment is a separate, firm line

The defence covers being over the THC limit. It does not cover impaired driving. Driving while your ability is affected is a separate offence under section 4 of the same Act, and a prescription is no defence to it. If your medicine makes you drowsy, slows your reactions, or affects your concentration, you must not drive. The responsibility for that judgement sits with you.

If you are stopped while driving

Roadside swabs detect the presence of THC, not impairment, so a positive swab does not by itself mean you are impaired. Tell the officer calmly that you are a lawfully prescribed patient, and show your prescription and your medicine in its original packaging. One important point: refusing to provide a sample when a police officer requires it is a separate criminal offence in its own right, so refusing can cause problems even where you would have had a defence.

Telling the DVLA

You must tell the DVLA if you have a condition, or take medication, that could affect your driving. Whether your situation needs reporting depends on your condition and how your medicine affects you, which is something to confirm with your specialist and the DVLA.

Where to get advice

This article explains the law. It does not tell you whether or when it is safe for you to drive, because that depends on your medicine, your dose, and how it affects you, none of which we can assess. Those are questions for your prescribing specialist and the DVLA. The Cannabis Industry Council has also published driving guidance for patients.

What this means for you: The law gives prescribed patients a defence for being over the THC limit, but only if the medicine is prescribed, taken as directed, and you are not impaired, and it never excuses impaired driving. Carry your prescription and original packaging, do not refuse a police sample, and tell the DVLA if your condition or medicine could affect your driving. If in any doubt, do not drive, and take the question to your specialist and the DVLA.

Sources: Road Traffic Act 1988, sections 4 and 5A; GOV.UK, drug driving; DVLA; Misuse of Drugs Regulations 2001.

Related: Stopped by the police with a prescription; Cannabis and your job; Is medical cannabis legal?

By The Plain Line. Last updated June 2026. This is information, not medical or legal advice. The rules change over time, so we date and review our guides.