Driving on a medical cannabis prescription

Driving while unfit. The government's guidance says: "It's illegal in England, Scotland and Wales to drive with legal drugs in your body if it impairs your driving." This applies whatever the drug is and however you came by it.

Driving over the limit. Section 5A of the Road Traffic Act 1988 makes it an offence to drive with more than a specified amount of certain controlled drugs in your blood. The limit for THC is 2 micrograms per litre of blood, which the government describes as a "zero tolerance approach". A patient taking a prescribed product that contains THC should assume they may be over it.

Section 5A gives a defence to the over-the-limit offence. You have to show all of these:

• the drug was prescribed or supplied to you for medical purposes;
• you took it as your prescriber directed, and as any instructions that came with it said;
• you held it lawfully.

The defence is lost if you went against advice about how long to wait between taking the medicine and driving. That advice can come from your prescriber or from the instructions with the medicine.

The defence does not cover driving while unfit. The government's guidance puts the two together: you can drive after taking a prescribed drug if you followed the advice on how to take it and it is "not causing you to be unfit to drive even if you're above the specified limits".

• Do not drive if you feel affected. If you are drowsy, slower to react, or not sure, do not drive. The guidance says to talk to your doctor, pharmacist or healthcare professional if you are unsure whether you should drive.
• Ask your prescriber how long to wait. Ask them directly how long you should leave between a dose and driving, and ask for the answer in writing. The defence depends on following that advice.
• Carry proof. The government's advice is: "If you drive and take prescription medicine, it may be helpful to keep evidence of this with you in case you're stopped by the police." A copy of your prescription, the medicine in its original labelled container, and a letter from your clinic all help.
• Expect a roadside test to show cannabis. The police can use a roadside kit that screens for cannabis. It cannot tell a prescribed medicine from an illegal drug. If the police think you are unfit to drive you can be arrested and asked for a blood or urine sample at a police station.

You must tell the DVLA if you develop a "notifiable" medical condition, or one you have gets worse. Notifiable conditions are "anything that could affect your ability to drive safely". The rule is about your condition, so check whether the condition you are being treated for is on the DVLA's list. You can be fined up to £1,000 for not telling the DVLA about a condition that might affect your driving.

A drug-driving conviction can mean a driving ban of at least one year, an unlimited fine, up to six months in prison and a criminal record. The conviction stays on your licence for 11 years.

The over-the-limit offence does not apply in Northern Ireland, but the government's guidance says you could still be arrested there if you are unfit to drive.

This guide is general information, not legal advice. If you are stopped or charged, speak to a solicitor.