1. The Legal Limits for THC in Driving

The UK has strict drug-driving laws, which set legal limits for controlled substances in the bloodstream. For THC (the psychoactive component in cannabis), the legal driving limit is 2 micrograms per litre of blood (µg/L). This is extremely low, meaning even small amounts of THC in your system could lead to a positive test.

How Does This Affect Medical Cannabis Users?

Most CBD-only products (such as Epidyolex) do not contain THC, so they are not subject to drug-driving limits.
Full-spectrum medical cannabis, which contains THC, can cause patients to exceed the legal driving limit even if they do not feel impaired.

2. The "Medical Defence" for Prescription Cannabis Users

The UK law does allow a medical defence for patients prescribed cannabis-based medicines. This means that if you are stopped by the police and test positive for THC, you may not be charged if you can prove:

• You have a valid prescription for a cannabis-based medicine.
• You are not impaired while driving.

Important Considerations

The medical defence does not mean automatic protection. If the police believe you are impaired, you can still be arrested and prosecuted, even with a prescription.

The burden of proof is on the driver to show they are using the medication legally and responsibly.

3. How the Police Test for Cannabis While Driving

If a driver is suspected of drug driving, the police may conduct the following tests:

1. Roadside Saliva Test (DrugWipe Test)
This test detects THC in saliva within minutes.
If the test is positive, the driver is taken to the police station for further testing.

2. Blood Test at the Police Station
A blood sample is taken to measure THC levels.
If the level is above 2 µg/L and the driver cannot prove they have a prescription and are unimpaired, they may be charged.

3. Field Impairment Test
The police may ask the driver to perform coordination tests, such as walking in a straight line or following a moving object with their eyes.
If the driver fails the test, they can be arrested, even if THC levels are below the legal limit.

4. Penalties for Drug Driving

If found guilty of drug driving, penalties include:

• Minimum 1-year driving ban
• Unlimited fine
• Possible prison sentence of up to 6 months
• Criminal record
• Higher car insurance costs
• Potential loss of employment (if driving is essential for work)
• A drug-driving conviction remains on your licence for 11 years, affecting future job opportunities and travel to some countries, like the USA.

5. Best Practices for Medical Cannabis Patients

1. Avoid Driving if You Feel Impaired
Even if you have a prescription, do not drive if you feel drowsy, dizzy, or cognitively impaired. Side effects such as delayed reaction times or drowsiness can increase accident risks.

2. Understand How Long THC Stays in Your System
THC can remain in the blood for hours or even days, depending on how it is consumed.

Vaporized cannabis has a shorter detection window (typically 4–6 hours).

Oral cannabis (oils, capsules, edibles) stays in the system much longer (up to 24 hours or more).

Regular or long-term users may have THC in their system even when they are no longer impaired.

3. Carry Your Prescription and Doctor’s Letter
If stopped by the police, having a copy of your prescription and a letter from your doctor can help prove you are using medical cannabis legally.

4. Plan Your Dosing Schedule
If you need to drive, time your doses carefully to ensure you are below the THC limit and not impaired.

5. Use CBD-Only Products if Driving Often
CBD products without THC (such as Epidyolex) do not affect driving laws and are a safer option for those who need to drive regularly.

6. Future Changes to UK Laws?
Some campaigners argue that the UK’s 2 µg/L THC limit is too low and does not account for medical cannabis users who are not impaired. Other countries, such as Germany and Canada, have higher THC limits or impairment-based testing rather than strict thresholds.

There have been discussions about updating UK laws to reflect the needs of medical cannabis patients, but no major legal changes have been made so far.

Conclusion

While medical cannabis is legal in the UK, driving with THC in your system remains legally risky. Patients can use the medical defence if they have a prescription and are not impaired, but they can still face penalties if they exceed the strict THC limit. To stay safe and legal, medical cannabis users should be cautious, plan their doses, and avoid driving if there is any risk of impairment.