THC Limits, Roadside Swabs and the Medical Defence: How Prosecutions Actually Work

For the plain-English overview of the law, start at medical cannabis and driving in the UK. This piece is the technical deep-dive: what the number actually is, how prosecutions are built, what the medical defence requires, and where the defence can fail.

Two offences, one stop

UK drug-driving law rests on two overlapping offences:

• Section 4 of the Road Traffic Act 1988 — driving while unfit through drink or drugs. This is an impairment offence. The prosecution has to prove your driving was impaired; the presence of a drug is only evidence.
• Section 5A of the Road Traffic Act 1988 — driving with a controlled drug in the body above a specified limit. This is a strict-limit offence. Once the concentration is above the limit, the offence is complete — impairment does not need to be proved.

Section 5A was introduced in 2015 alongside The Drug Driving (Specified Limits) (England and Wales) Regulations 2014. The regulations set limits for 17 drugs; the eight "illicit-use" drugs (including delta-9-THC) got deliberately low limits, and the nine "medicinal" drugs got higher, therapeutic-range limits.

The 2 µg/L limit

The delta-9-THC limit is 2 micrograms per litre of blood. The Department for Transport's own explanatory memorandum described this as an "accidental exposure" limit — chosen to allow prosecution of any deliberate cannabis use while making it unlikely that a genuinely passive-exposure result would exceed it. It was not chosen as a threshold for impairment. There is no equivalent to the alcohol curve, where 80 mg/100 ml has a broadly predictable behavioural effect; 2 µg/L of THC in blood can coincide with anything from full impairment (recent smoker) to none at all (regular medical user, dosed hours ago).

This is what makes Section 5A structurally hostile to prescribed patients: therapeutic doses of medical cannabis routinely produce blood THC concentrations well above 2 µg/L for many hours after a dose, long after any subjective impairment has passed. The strict-limit design would criminalise every prescribed patient every time they drove — except for the medical defence.

The statutory medical defence

Section 5A(3) of the Act creates a defence to the strict-limit offence. It has three elements, all of which must be satisfied:

1. The drug was prescribed or supplied to you for medical or dental purposes.
2. You took the drug in accordance with any directions given by the person who prescribed or supplied it and with any accompanying instructions given by the manufacturer.
3. Your driving was not impaired as a result of taking the drug.

The burden is on the defendant to raise the defence on the balance of probabilities (a lower standard than the criminal standard of "beyond reasonable doubt"), after which the prosecution must disprove it beyond reasonable doubt. The Section 5A(3) defence does not apply to the Section 4 impairment offence — for that, the prosecution has to prove impairment in the first place, so no defence is needed.

How the process actually runs

1. Roadside

An officer with reasonable suspicion — behaviour, driving, admission, smell — administers a preliminary drug test. In practice this is almost always a Draeger DrugWipe or similar oral fluid device, screening for cannabis and cocaine. A positive result is a screening indication only, not evidence for court. Refusing this test is an offence under Section 6A.

2. Arrest and station

A positive DrugWipe usually results in arrest and transport to a police station. There, a Health Care Professional (usually a nurse or forensic physician) takes an evidential blood sample, in two portions — one for the FSP, one preserved for the defence to have independently analysed.

This is the point where you tell the custody officer, on record, that you are a prescribed patient. Ask for it to be noted. Ask for your prescription documents to be brought to you or copied from your phone. It does not stop the process — the sample still goes to the lab — but it puts your defence on the custody record from the earliest moment. If you're asked to provide a sample, do it. Refusal is a separate offence under Section 7 that generally carries the same penalties as the substantive offence, with no medical defence.

3. Laboratory

The Forensic Service Provider quantifies delta-9-THC (and 11-nor-9-carboxy-THC, a metabolite, for corroboration) using LC-MS/MS. Results are usually available in 4–8 weeks, sometimes longer.

4. Charging decision

If the delta-9-THC reading exceeds 2 µg/L, the file goes to the CPS. The CPS applies the Full Code Test: (a) is there sufficient evidence for a realistic prospect of conviction, and (b) is prosecution in the public interest. A well-documented medical defence — clinic letter, current repeat, honest custody-record entry — can and does result in the case not being charged, or being discontinued after charge.

5. Court (if charged)

The case is heard in the Magistrates' Court. Your solicitor raises the Section 5A(3) defence with your medical records, prescription, dosing history and (usually) expert pharmacology evidence about the relationship between your dose and expected blood concentrations. The prosecution rebuts with the officer's observations of impairment and any admissions from interview. This is why not admitting impairment at the roadside matters — a recorded admission is very hard to walk back.

Where the medical defence can fail

• Non-prescription source — buying flower from a non-clinic source, even if you also have a legitimate prescription, breaks element 1 for that dose.
• Dose above direction — self-dosing higher or more frequently than your clinic titrated to breaks element 2.
• Recorded impairment — officer observations of red eyes, delayed responses, or an admission of "I had one this morning and feel a bit foggy" all support element 3 failing.
• Combined substances — alcohol, illicit drugs, or non-prescribed medications on top of the CBPM change the impairment picture and undermine the "not impaired as a result of the prescribed drug" element.

Practical implications for prescribed patients

• Carry your repeat prescription, dispensing label and the most recent clinic letter every time you drive. Digital copies on your phone plus a printed copy in the car is the standard.
• Do not drive within the window your clinic told you to avoid — even if you feel fine. A recorded direction from your clinic is part of element 2.
• If stopped, disclose the prescription proactively and ask for it to be noted. Do not admit impairment.
• Never combine the CBPM with alcohol before driving. It is the single most common defence-killer.

For the roadside stop scenario in plain language, see stopped by police with a cannabis prescription.