Driving - PatientsCann UK®

Do not refuse a roadside drug swab or blood test without a medical reason

Refusing to give a sample when a police officer asks for one is a separate crime. This is true even if you have a medical cannabis prescription. Even if you would have had a valid defence against a drug driving charge, refusing a sample can still get you charged, and it can weaken your defence. Only refuse if you have a real medical reason, and tell the officer clearly. Always get legal advice as soon as you can if you are stopped.

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Use the tabs below to move between topics. Read each part with care, above all “Police Stop” and “Drug Testing”. The law is not the same across the UK, so pick your nation above to see the rules that apply to you.

The Law: What Applies to You

UK drug driving law, the legal limit for THC, and the medical defence that protects patients. The law is not the same across the UK, so this section changes with the nation you pick above.

There is a set THC limit: Section 5A Road Traffic Act 1988

It is an offence to drive with more than a set amount of a controlled drug in your blood. For cannabis (THC) the limit is 2 micrograms per litre of blood.1 This is a very low limit. Even if you did not use cannabis just before driving, THC can stay in your blood for several days and push you over it. In Scotland the limit is the same, set by its own regulations.2

Read Section 5A RTA 1988

THC is in the “zero tolerance” group

Cannabis (THC) is a “zero tolerance” drug for driving. This means the limit is set very low, not at a level that shows you are impaired. The same group includes cocaine, MDMA, LSD, ketamine, heroin, and methylamphetamine. The key point for patients is that a medical defence is available for THC if you meet the conditions below.

The medical defence

Even if you test over the limit, you have a valid defence under s.5A(5) RTA 1988 1 if all three of these are true:

All three must be true: the medical defence

The medicine was prescribed to you
You took it according to your prescriber's instructions
It was not affecting your ability to drive safely at the time

The police must prove their case, but a defence is not a guarantee

Being over the limit does not always lead to a conviction where the medical defence applies. But you must still cooperate fully. Refusing a test badly weakens your position and is a separate crime. Always get legal advice quickly.

Northern Ireland has no set THC limit

The Road Traffic Act 1988 and its 2 microgram limit do not apply in Northern Ireland. There is no set THC limit here and no “specified limit” offence. Instead, the law is about whether your driving was impaired.

The offence is driving while unfit through drugs

Under Article 15 of the Road Traffic (Northern Ireland) Order 1995, it is an offence to drive, or try to drive, while unfit through drink or drugs.3 The test is whether your ability to drive was actually harmed. The police must show that your driving was impaired.

Your prescription is strong evidence, not an automatic defence

There is no s.5A-style medical defence in Northern Ireland, because there is no set-limit offence to defend against. But a lawful prescription, taken as your prescriber told you, is strong evidence that you were not impaired. The safe rule stays the same: do not drive if your medicine affects your driving.

You must still cooperate with the police

Refusing a test is a separate crime in Northern Ireland too. Always give any sample the police ask for, and get legal advice quickly. Tell the officer about your prescription and show your medicine in its original box.

Home Office advice to prescribers: when they prescribe THC-containing medicine, patients must be told that the medicine is likely to affect their driving, that they should not drive until they know how it affects them, and that driving while impaired is a crime even with a prescription.

Should I Drive After Taking My Medication?

Understanding impairment, personal tolerance, and when it is safe to get behind the wheel.

No one should drive while impaired

No one should drive while impaired, whether or not they have a prescription. This is true for any substance that affects your ability to drive safely. This is your personal responsibility. A prescription does not let you drive impaired. It gives you a possible legal defence if you are tested and were not impaired.

THC limits & individual tolerance vary

Being over the 2 microgram limit does not automatically mean you are impaired. THC affects people differently, based on tolerance, how your body works, the time since your last dose, the product, and the amount.

Northern Ireland has no set THC limit, so you are not judged on a blood level. What matters is whether your driving is impaired. THC affects people differently, based on tolerance, how your body works, the time since your last dose, the product, and the amount.

But you must be honest with yourself. If you feel drowsy, unfocused, or slower to react, do not drive.

Know your own medication

Products that are mostly CBD, with little or no THC, are unlikely to affect your driving. Products with more THC carry a higher risk. Do not drive until you know how your own medicine affects you, with your prescriber's guidance. Some patients on a steady, long-term dose may feel confident driving. Others on high THC doses should not drive at all.

Consider timing of your doses

THC can remain detectable for days, but acute impairment typically peaks shortly after consumption and reduces. Talk to your prescriber about scheduling doses to allow driving when needed. Evening dosing may be preferable for patients who need to drive during the day.

Ask your prescriber

Always discuss driving with your prescribing clinician when starting or changing medication.

Know your side effects

Drowsiness, dizziness, or delayed reactions are a sign you should not drive.

Time your doses

Plan your dosing schedule around when you need to drive, in consultation with your clinic.

When in doubt, don't drive

If you feel any uncertainty about your fitness to drive, wait or arrange alternative transport.

If You Are Stopped by Police While Driving

Step-by-step help on how to handle a roadside stop calmly and correctly as a medical cannabis patient.

Stay calm and comply with the stop

Pull over safely, switch off the engine, and remain polite and cooperative throughout. Do not argue, raise your voice, or become confrontational. Your attitude during a stop can influence how the encounter proceeds.

Inform the officer you are a medical cannabis patient

Clearly and calmly tell the officer that you hold a valid UK medical cannabis prescription and that you have your medication with you. Use clear language: "I hold a valid private medical cannabis prescription in the UK. I am happy to show you my documentation."

Show them your medication in its original packaging with the dispensing label, and ideally a clinical letter or copy of your FP10. This information is relevant to whether the medical defence applies.

Do NOT refuse a roadside drug swab, unless you have a medical reason

This is important. Refusing to give a sample is a separate crime.

In England, Wales and Scotland this is an offence under Section 6 of the Road Traffic Act 1988.4 You can be charged for refusing even if you would have had a full medical defence against a drug driving charge.

In Northern Ireland this is an offence under Article 18 of the Road Traffic (Northern Ireland) Order 1995.5 You can be charged for refusing even if you had a strong case that you were not impaired.

If you have a real medical reason that stops you giving a sample (for example a clotting disorder, a documented needle phobia, or being unable to blow), tell the officer clearly and calmly, and they should write it down. Being medically unable to give a sample is not the same as refusing.

Refusing without a medical reason is a crime. It can lead to an automatic charge, a fine, and possible loss of your licence, and it takes away your ability to rely on a medical defence in court.

Note everything down after the encounter

As soon as it is safe to do so, write down the time, location, officers' badge numbers, what was said, what tests were administered, and any other relevant details. This information can be vital if the matter proceeds to court.

Seek legal advice immediately if arrested or charged

If you are arrested, you have the right to free, independent legal advice from a duty solicitor at the police station. Always ask for a solicitor before you answer any more questions. Contact our legal support resources once you are free to do so.

PatientsCann UK legal support resources

Common Roadside Scenarios

An officer may tell you that your prescription “doesn't matter” or cannot be used as a defence. Do not argue. Stay calm. Politely say that you understand you may be over the limit, but that you have a valid prescription and plan to raise the medical defence under s.5A(5) RTA 1988 if needed. Do not resist any testing. The officer's job is to gather evidence. The defence is decided later by the courts, not at the roadside. You can say: “I understand. I have my prescription documents and will cooperate fully. I would like to note that I plan to raise the medical defence.”

This is a hard situation, but the prescription still exists. You just cannot prove it at the roadside. Still cooperate fully with all testing. Do not refuse any tests. Tell the officer you are a patient and that you can provide documents. Ask if you can contact your clinic or open a digital copy of your prescription. Once you are home or somewhere safe, contact your clinic straight away for a letter and gather your documents. If you are charged, your legal representative can send the prescription and medical records as evidence. From now on, always carry your documents with your medicine.

It is highly recommended to keep a printed clinical letter and a copy of your FP10 in your vehicle at all times alongside your medication.

You do not have an absolute right to speak to a solicitor before you give a roadside breath, oral fluid (swab), or preliminary impairment test. These are preliminary roadside checks. But once you are arrested and taken to a police station, you have an absolute right to free, independent legal advice. Always use it. At the station, say clearly: “I wish to speak to a solicitor before I answer any questions or give any more samples.” A duty solicitor is available 24 hours a day and is completely free.

Roadside Drug Testing: Swabs & Blood Samples

What happens when police use a roadside drug testing device, what a positive result means for patients, and why cooperation is essential.

The Drugwipe / Oral Fluid Test

Police usually use an oral fluid screening device (such as the Dräger DrugTest 5000 or the Cozart RapiScan) to test for drugs at the roadside. This means a swab is taken from the inside of your mouth. If you are prescribed THC-containing medicine, you should expect to test positive for cannabis, even if you did not use it that day. THC can show up in oral fluid for up to 24 hours after use, and sometimes longer.

A positive roadside result is not a conviction

A positive oral fluid test is a first screening only. It gives police fair grounds to arrest you and take you to a police station for a blood sample. The roadside result cannot be used in court by itself. Only the lab-tested blood result can be the basis of a prosecution.

A positive blood result over 2 micrograms per litre still does not always lead to a conviction where a valid medical defence applies.

In Northern Ireland there is no set blood limit. A conviction needs proof that your driving was impaired, and a lawful prescription taken as directed is strong evidence that it was not.

Always cooperate, do not refuse

Refusing to give an oral fluid sample at the roadside, or a blood sample at the station, is a crime.

This falls under Section 6 of the Road Traffic Act 1988.4

In Northern Ireland this falls under Article 18 of the Road Traffic (Northern Ireland) Order 1995.5

The most you can be punished for refusing is the same as for drug driving itself: an unlimited fine, a driving ban, and a criminal record.

This matters: refusing a sample does not protect you. If you refuse, you cannot use the medical defence. Courts have said again and again that refusal removes the ability to argue medical need.

Do not refuse a swab or blood sample. Always give the sample. Raise your prescription and the medical defence after the test, not instead of it.

The only exception: real medical inability

The law accepts that some people are medically unable to give a sample, not just unwilling. Examples include a diagnosed severe needle phobia, being unable to give enough oral fluid because of medication side effects, or a serious medical condition. If this applies to you, say so clearly and calmly to the officer: “I am medically unable to give this sample because of [condition]. I am not refusing.” The officer should write this down. Follow up with medical evidence as soon as you can. Talk to your prescriber in advance if you think this may apply to you.

At the station: the evidential blood sample

If you are arrested, you will be asked to give a blood sample at the police station. A medical professional takes it. The sample is split into two. One part is sent to a lab for testing, and one part is offered to you. Always accept your part of the sample. This lets your legal team have it tested on their own, which can be important for your defence. Again, do not refuse to give this sample without a documented medical reason.

Always request the duty solicitor at the police station before answering any questions. Access our legal support resources for further guidance and specialist solicitor contacts.

The Field Impairment Test (FIT)

What the FIT is, when police can request it, your right to refuse it, and the critical information medical cannabis patients must declare.

What is the Field Impairment Test?

Pupil measurement: An officer measures the size of your pupils with a pupillometer. Cannabis (THC) can cause pupil dilation, which may indicate recent use.

Romberg balance test: You are asked to stand upright, tilt your head back slightly, and estimate 30 seconds internally. Tests balance and time perception.

Walk and turn test: You walk heel-to-toe along a straight line, turn, and return. Tests balance, co-ordination, and the ability to follow instructions while performing a physical task.

One-leg stand test: You stand on one leg and count. Tests balance and physical co-ordination.

Finger to nose test: You are asked to touch your fingertips to the tip of your nose with your eyes closed and arms outstretched. Tests fine motor control and proprioception.

You can refuse a FIT, but understand what it means

Unlike refusing a drug swab or blood sample, refusing to do a Field Impairment Test is not a crime. You have the right to say no to a FIT. But be aware that a refusal can be noted by the officer and may be used as evidence that you did not cooperate. If you choose to refuse, do it calmly and clearly: “I decline to take part in the Field Impairment Test.” You do not need to give a reason, but you may choose to explain any relevant medical conditions.

Unlike refusing a drug swab, refusing a FIT is not a crime. You will not be charged for saying no to it. But it is different from a drug sample, so do not confuse the two.

Declare medical conditions that may affect your FIT performance

This is very important. Many medical conditions, completely unrelated to drug use, can affect how well you do FIT tasks. Before the test begins, clearly tell the officer about any conditions that may affect your performance. These include, but are not limited to:

Balance & inner ear conditions

Vertigo, BPPV, Ménière's disease, labyrinthitis, or other vestibular disorders affect balance tests.

Neurological conditions

MS, Parkinson's disease, cerebellar ataxia, peripheral neuropathy, or prior strokes affect co-ordination and balance.

Musculoskeletal conditions

Arthritis, joint replacements, lower limb injuries, or chronic pain conditions affecting standing or walking.

Eye conditions

Nystagmus (involuntary eye movement), glaucoma, or other eye conditions can directly affect pupil measurement results.

ADHD or autism

Attention and executive function differences can affect performance on divided attention tasks.

Other medications

Many prescribed medications (benzodiazepines, opioids, antihistamines, antidepressants) can affect test performance independently of cannabis.

Declare relevant conditions before the FIT begins. Say: "Before this test, I need to inform you that I have [condition], which may affect my performance on some of these tasks. This is separate from any medication I have taken." Ask the officer to record this declaration. If you have supporting medical documentation, tell them you can provide it.

FIT results & what happens next

If a trained Drug Recognition Expert (DRE) or officer trained in FIT decides there are signs of impairment, this, together with a positive drug test, can support a decision to arrest you and ask for a blood sample at the station. The FIT result is only one piece of evidence. It does not decide guilt on its own. Your declared conditions, your prescription history, and your blood test results all form part of the full picture the Crown Prosecution Service looks at.

If you are concerned about FIT performance due to a medical condition, speak to your prescribing clinic in advance. They can provide a letter documenting relevant conditions, which you should carry with your prescription documentation.

Sources

Road Traffic Act 1988, c. 52, s. 5A. Available at: https://www.legislation.gov.uk/ukpga/1988/52/section/5A (Accessed: 14 July 2026).
The Drug Driving (Specified Limits) (Scotland) Regulations 2019 (SSI 2019/236). Available at: https://www.legislation.gov.uk/ssi/2019/236 (Accessed: 14 July 2026). In England and Wales the limit is set by the Drug Driving (Specified Limits) (England and Wales) Regulations 2014 (SI 2014/2868).
The Road Traffic (Northern Ireland) Order 1995 (SI 1995/2994 (N.I. 18)), art. 15. Available at: https://www.legislation.gov.uk/nisi/1995/2994/contents (Accessed: 14 July 2026).
Road Traffic Act 1988, c. 52, ss. 6 to 7 (preliminary tests and provision of specimens). Available at: https://www.legislation.gov.uk/ukpga/1988/52/section/6 (Accessed: 14 July 2026).
The Road Traffic (Northern Ireland) Order 1995 (SI 1995/2994 (N.I. 18)), art. 18. Available at: https://www.legislation.gov.uk/nisi/1995/2994/contents (Accessed: 14 July 2026).
Department for Transport (2024) Drug driving and medicine: advice for healthcare professionals. GOV.UK. Available at: https://www.gov.uk/government/publications/drug-driving-and-medicine-advice-for-healthcare-professionals (Accessed: 14 July 2026).

The information on this page is for general informational purposes only and does not constitute legal advice. PatientsCann UK advises all patients to adhere to current UK laws and regulations regarding driving. It is your responsibility to ensure you are fit to drive. PatientsCann UK will not be liable for any consequences arising from the use of medical cannabis while driving. Always seek independent legal advice if stopped, arrested, or charged.