No question generates more anxiety among prescribed patients than driving. The concern is understandable: cannabis is a controlled substance, drug-driving is a criminal offence, and the consequences of getting it wrong include a driving ban, a criminal record, and — in some cases — a prison sentence.
The legal position is more nuanced than the headlines suggest. But nuance does not always survive a roadside encounter.
The Law: Section 5A and the Medical Defence
Since 2015, Section 5A of the Road Traffic Act has made it an offence to drive with certain controlled drugs in the blood above specified limits. For THC, the legal limit is 2 micrograms per litre of blood — a threshold so low that many prescribed patients exceed it days after their last dose.
However, Section 5A(3) provides a statutory medical defence. A driver has a defence if the drug was prescribed or supplied for medical purposes, taken in accordance with medical instructions, and did not impair their driving. This defence was confirmed in the House of Commons during the passage of the legislation and has been upheld in several magistrates' court decisions.
The Reality: What Happens at the Roadside
The medical defence exists in law. Whether it protects you in practice depends on what happens during a police encounter, and that introduces significant variability.
If stopped, you may be asked to perform a field impairment test. If the officer suspects impairment, you may be required to provide a blood sample at a police station. If the sample exceeds the 2μg/L THC limit, you will be charged. And then — and only then — does the medical defence become relevant, as part of your defence in court.
The problem is the gap between arrest and acquittal. Even if you are ultimately found not guilty, you may spend months with the case hanging over you, your licence suspended, and your name associated with a drug-driving charge. For many patients, the legal defence works — but at a cost that the law does not compensate.
What Patients Should Do
Carry your prescription documentation with you whenever you drive. This should include your clinic letter, your dispensing label, and — ideally — a letter from your clinician confirming that your prescribed medication does not impair your ability to drive.
Do not drive if you feel impaired. The medical defence only applies if your driving was not impaired. If you have just taken your medication and feel sedated, dizzy, or otherwise unable to drive safely, the defence will not protect you — and it shouldn't. The law expects patients to make responsible decisions about their own fitness to drive, just as it does for patients prescribed opioids, benzodiazepines, or any other medication that can affect cognition.
Be aware that the blood THC limit is low. Many patients who take THC-containing medication daily will exceed 2μg/L even when they last medicated 12 or 24 hours earlier. The medical defence exists precisely for this reason — the limit was designed for recreational use, not therapeutic maintenance. But until roadside saliva testing for impairment (rather than mere presence) becomes standard, the risk of arrest remains real even when the risk of conviction is low.
Where Reform Is Needed
The current law creates a paradox: it is legal to take prescribed cannabis and legal to drive, but the enforcement mechanism does not distinguish between a patient maintaining a therapeutic baseline and a recreational user who has just consumed. Reform efforts, led by organisations including the Medical Cannabis Clinicians Society, are pushing for impairment-based testing rather than presence-based limits. Until that happens, patients will continue to navigate a legal grey zone that the law was explicitly designed to address — but which the roadside reality has yet to catch up with.
