Driving on a cannabis-based medicine.

What is the drug-driving limit for THC?

In England, Wales and Scotland, the limit for THC is two micrograms per litre of blood. It's an offence to drive, or be in charge of a vehicle, with more than that in your blood. THC is the main active ingredient in most cannabis-based medicines, and the limit is set so low that someone who has taken their medicine recently may well be over it. Northern Ireland doesn't have this specific offence, but it does have the impairment one below, so don't assume you're in the clear there either.

Separately, and everywhere in the UK, it's an offence to drive while unfit through drugs. That one is about impairment: whether the drug has affected your ability to drive properly. It has no medical defence.

In Scotland, how it works in Scotland covers what happens after a positive roadside test.

What is the medical defence?

It's a defence in law to the over-the-limit offence, for people with a legitimate prescription. You can rely on it if the medicine was prescribed to you for medical purposes, you took it in accordance with the instructions of the prescriber and any information supplied with it, and you weren't driving impaired. If you're charged and you raise this defence with some evidence, it's then for the prosecution to prove it doesn't apply.

Read that middle condition again: in accordance with the instructions. If your clinic has told you not to drive for a set time after a dose, or not to drive at all while you're settling on a new product, and you've ignored that, the defence is weaker. Follow the plan.

Impaired is impaired

This is the part I'd underline twice. A prescription doesn't make you safe to drive. If you feel drowsy, slowed, foggy or dizzy, you're impaired, and the medical defence does not cover you. Everyone is different, products are different and the first weeks on a new medicine are the riskiest. Most clinics advise not driving for several hours after a dose, and many advise not driving at all until you've been stable on a dose for a while. Your clinic's advice is the advice to follow, and if they haven't given you any, ask for it in writing.

What happens if the police stop me?

Police can require a roadside saliva test, which will very likely show THC. That's not the end of the story. Be calm and polite, tell the officer straight away that you have a prescribed cannabis-based medicine, and show your evidence: the original packaging with the pharmacy label, a copy of your prescription, and ideally a letter from your clinic. You may still be arrested and taken for a blood test, which is the sample the law is based on. If it comes to a charge, the medical defence is dealt with through the legal process, so get legal advice rather than relying on the roadside conversation.

Visiting from abroad and hiring a car? The same law applies to you. See the guide for visitors to the UK.

What helps enormously, in my experience, is having that paperwork in the car before you ever need it. Photos on your phone of the prescription and the packaging are better than nothing; the real thing is better still.

In Scotland, read what applies if the police stop you in Scotland.

Do I need to tell the DVLA or my insurer?

Usually not about the medicine itself, but you may need to tell the DVLA about your condition. There's no general duty to tell the DVLA you've been prescribed a cannabis-based medicine. You do have to tell it about any medical condition that affects your driving. Some of the conditions people are prescribed cannabis-based medicines for are on that list, so it's worth checking on the DVLA website. Your doctor may also advise you not to drive at certain stages, and the DVLA expects you to follow medical advice. Most insurers don't need to be told about a prescribed medicine, but if in doubt, check your policy; being uninsured is a far bigger problem than an awkward phone call.

Keep it simple

Take your medicine as prescribed. Don't drive if you feel any effect at all. Keep your paperwork with you. Follow the clinic's timing advice, especially early on. And if any of that feels unclear, ask your clinic to put its driving advice in writing, because that piece of paper is your best friend on a bad day.