About 47% of the UK adult population takes at least one prescribed medication. Most of those people have no idea that their doctor's prescription could lead to a criminal conviction for drug driving.
The law in England and Wales sets blood concentration limits for eight prescription drugs. If you exceed those limits while driving, you commit a criminal offence carrying the same penalties as someone caught driving on cocaine or cannabis, a minimum 12-month driving ban, an unlimited fine, and a criminal record.
There is a defence. Section 5A(3) of the Road Traffic Act 1988 protects drivers who take their medication exactly as prescribed. But this defence is narrower than it sounds, and the burden of proving it falls on you.
The Drug Driving (Specified Limits) (England and Wales) Regulations 2014 set blood concentration limits for 17 controlled substances. Eight of those are prescription medications. The remaining nine are illegal drugs like cannabis and cocaine, which carry near-zero limits under a “zero tolerance” approach.
For prescription drugs, the government used a “road safety risk-based approach.” The limits sit above normal therapeutic blood concentrations, so that a patient taking the standard dose as prescribed should not normally breach the threshold. That word “normally” matters. Dose adjustments, individual metabolism, drug interactions and simple biological variation mean some patients exceed the limit while following their doctor’s instructions to the letter.
This is a strict liability offence. The prosecution does not need to prove you were impaired, drowsy or driving badly. If the blood test shows a concentration above the limit, you’ve committed the offence. The only escape route is the medical defence.
Our drug driving solicitors handle prescription medication cases across England and Wales.
The specified limits for prescription medications (in micrograms per litre of blood):
Drug | Common brand names | Limit (µg/L) | Commonly prescribed for |
|---|---|---|---|
Clonazepam | Rivotril | 50 | Epilepsy, anxiety |
Diazepam | Valium | 550 | Anxiety, muscle spasms, alcohol withdrawal |
Flunitrazepam | Rohypnol | 300 | Severe insomnia (rarely prescribed in UK) |
Lorazepam | Ativan | 100 | Anxiety, pre-operative sedation |
Oxazepam | — | 300 | Anxiety (metabolite of diazepam) |
Temazepam | — | 1,000 | Insomnia |
Methadone | Methadose, Physeptone | 500 | Opioid dependency |
Morphine | MST, Oramorph, Sevredol | 80 | Severe pain |
Amphetamine (limit: 250µg/L) sits in both categories because it is prescribed for ADHD under names like Dexedrine and Elvanse, while also being a recreational drug.
Notice the disparity between these limits and the illegal drug limits. Cannabis (THC) has a limit of just 2µg/L. Cocaine’s metabolite is capped at 50µg/L. The prescription limits are deliberately higher to allow therapeutic use, but “higher” does not mean “impossible to exceed.”
The official government guidance on drug driving regulations explains the rationale behind each threshold.
Diazepam, lorazepam, clonazepam, temazepam and oxazepam are all benzodiazepines. They are the most common prescription drugs involved in drug driving prosecutions.
Benzodiazepines cause drowsiness, slowed reactions, impaired coordination and poor concentration. Their effects last longer than many patients expect. Diazepam has an active metabolite (desmethyldiazepam) with a half-life of up to 100 hours, meaning the drug’s effects can persist for days after a single dose.
If your GP has recently started you on a benzodiazepine, increased your dose, or switched you from one benzodiazepine to another, the first few days carry the highest risk of exceeding the legal limit. This is exactly the period when your doctor might tell you to “see how you get on” — while the law says you could be committing a criminal offence every time you drive.
Our page on what happens if you’re caught drug driving covers the process from the moment of being stopped.
Morphine has a limit of 80µg/L. Codeine is not listed as a specified drug, but your body converts codeine into morphine during metabolism. So a patient taking codeine (including over-the-counter co-codamol) can trigger a blood test positive for morphine above the limit.
This catches people off guard. You can buy Co-Codamol from a pharmacist without a prescription. Taking the recommended dose of a non-prescription painkiller, then driving, can put you above the legal limit for morphine. If you’re taking higher-strength codeine on prescription, 30mg or 60mg tablets, the risk is higher still.
Tramadol is not a specified drug, but it is a controlled substance that can cause impairment. A driver whose blood test comes back negative for the specified drugs could still face prosecution under Section 4 of the Road Traffic Act 1988 (driving while impaired) if tramadol affected their ability to drive.
Patients on methadone programmes for opioid dependency have a limit of 500µg/L. Dose adjustments are common during the stabilisation phase of treatment. If your dose has recently been increased, your blood methadone levels will be higher than usual and may exceed the threshold temporarily.
The difficulty with methadone is that patients often drive to collect their daily dose from a pharmacy. If the police stop you on the way home, and your blood level is above 500µg/L, you’re in trouble regardless of how safely you were driving.
Section 5A(3) of the Road Traffic Act 1988 gives you a statutory defence if you can prove two things:
If both conditions are met, you have a complete defence, even if your blood test showed levels above the specified limit.
This sounds straightforward. It is not.
The Crown Prosecution Service guidance on drug driving sets out when the medical defence fails:
Taking the wrong dose. If your prescription says “one tablet twice daily” and you took two tablets because the pain was bad, the defence does not apply. You were not following the prescriber’s directions.
Combining with alcohol. If your medication leaflet says “avoid alcohol” and you had a glass of wine, the defence is lost. The manufacturer’s instructions and the prescriber’s advice form part of the requirements.
Using someone else’s medication. The drug must be prescribed, dispensed or supplied to you specifically. Taking your partner’s diazepam because you ran out of your own prescription is not covered.
Old prescriptions. If your GP stopped prescribing the medication six months ago but you still had tablets left and took them, there is a question about whether the drug was currently “prescribed” to you. The CPS may argue it was not.
The burden of proof sits with the defendant. You must show, on the balance of probabilities, that both conditions are met. In practice, this means you need:
Our prescription drug driving defence page covers these strategies in more detail.
Prescription drug driving carries the same penalties as any other drug driving offence. The Sentencing Council guidelines apply equally whether the substance is diazepam or cocaine:
First offence:
Second offence within 10 years:
Aggravating factors that push the sentence higher include causing an accident, having passengers in the vehicle, driving for hire, and being significantly above the specified limit.
The endorsement code is DR80 (drug/driving — specified drug above specified limit). It remains on your licence for 11 years. For more on endorsement codes, see our DR10 driving offence guide.
Beyond the court sentence, a conviction hits your finances and career. Insurance premiums increase sharply — some insurers won’t cover you at all. If you drive for work, a 12-month ban means 12 months without income. Professional regulators in healthcare, finance and law may take disciplinary action against members convicted of drug driving.
Police can require a preliminary drug test if an officer suspects you have consumed drugs or committed a moving traffic offence. They do not need to prove impairment at this stage — reasonable suspicion is enough.
The officer may carry out a field impairment assessment at the roadside. This includes:
These tests are designed to identify signs of drug impairment. They are not foolproof. Nervousness, medical conditions, fatigue and poor coordination can all produce results that look like drug impairment when they are nothing of the sort.
The DrugWipe device tests saliva for cannabis (THC) and cocaine only. It does not detect benzodiazepines, opioids or any other prescription drugs. So if you are stopped on suspicion of driving under the influence of prescription medication, the roadside swab will come back negative. The officer must then rely on the field impairment assessment and their own observations to justify the arrest and a blood test.
If the officer has sufficient suspicion, you will be arrested and taken to a police station, where a healthcare professional will take a blood sample. This must follow PACE 1984 requirements.
The sample is split into two vials — one for the prosecution’s laboratory, one for you to have independently tested. Refusing to provide blood without a reasonable excuse is a separate offence carrying the same penalties as the drug driving charge itself.
Blood test results take 4-8 weeks. You will usually be released on bail in the meantime.
The blood collection, storage and analysis process must follow strict protocols. Our solicitors examine:
Errors in this chain are not rare. Blood stored at room temperature degrades. Mislabelled samples cannot be attributed to you. Laboratories that fail accreditation inspections produce results the court cannot rely on.
As explained above, Section 5A(3) protects patients who were following their prescription. Building this defence requires documentation: your prescription, pharmacy records, GP notes confirming the dose, and the patient information leaflet. Our solicitors gather this evidence systematically.
If your reading was close to the limit, the laboratory’s margin of error may be your defence. All analytical methods have inherent uncertainty. A result of 560µg/L for diazepam (limit: 550µg/L) may actually represent a true level below the threshold when the margin of error is factored in.
Your retained blood sample can be sent for independent analysis. Where the prosecution’s result and your independent result differ, the court must decide which is more reliable.
Some combinations of prescription drugs cause blood levels to rise unpredictably. If your GP prescribed two medications that interact, for example, a benzodiazepine and an opioid, and one drug slowed the metabolism of the other, your blood level may have exceeded the limit through no fault of your own. Expert pharmacological evidence can support this defence.
If you take any of the eight specified prescription drugs and you drive, take these steps now, before you end up in a police station:
If you’re already taking medication and concerned about your legal position, our drug driving solicitors can assess your risk in a free consultation.
Can I drive on diazepam?
You can drive if your blood diazepam level is below 550µg/L and you are not impaired. If you take diazepam exactly as prescribed and at a standard therapeutic dose, your levels should normally stay below the limit. But dose changes, individual metabolism and interactions with other drugs can push you over. If you’ve recently started diazepam or had your dose increased, avoid driving for the first few days.
Can I drive on codeine?
Codeine is not a specified drug, but your body converts it into morphine, which has a limit of 80µg/L. Regular use of codeine painkillers (including over-the-counter co-codamol) can produce blood morphine levels above this threshold. If you take strong codeine regularly, get advice before assuming you can drive.
Is drug driving with prescription medication a criminal offence?
Yes. It carries the same penalties as driving on illegal drugs: a minimum 12-month ban, unlimited fine, possible community order or imprisonment, and a criminal record. The only difference is that the legal limits are set higher for prescription drugs, and the medical defence is available.
What is the medical defence for prescription drug driving?
Section 5A(3) of the Road Traffic Act 1988 provides a complete defence if you can prove the drug was prescribed to you and you took it as directed. Both conditions must be met. The burden of proof is on you.
Will my doctor’s letter protect me?
A letter from your GP saying you’re fit to drive can support the medical defence, but it is not a guarantee. The court examines whether you were following the prescription, not whether your doctor thought you were safe to drive. A GP letter, pharmacy records plus the patient information leaflet together build a strong defence.
Can I be convicted if I was driving safely?
Yes. Under Section 5A, the offence is committed when your blood level exceeds the specified limit, regardless of your driving standard. The prosecution does not need to prove impairment.
What happens if I refuse the blood test?
Refusing without a reasonable excuse is a separate offence under Section 7 of the Road Traffic Act 1988. It carries the same maximum penalties as the drug driving charge. Recognised reasonable excuses include a genuine needle phobia supported by medical evidence.
Should I tell the police about my prescription at the roadside?
Tell the officer you take prescribed medication, but do not go into detail about the case. Provide the basic information required by law and save the detailed discussion for your solicitor. What you say at the roadside can be used as evidence.
How long does it take to get blood test results back?
Typically 4-8 weeks. During this time, you can usually continue driving unless a specific bail condition prohibits it.
Can I get a shorter ban for prescription drug driving?
The minimum ban is 12 months for a first offence. Courts have limited discretion to reduce this. However, mitigation, particularly evidence that you were taking medication as prescribed and acting in good faith, can influence the fine level, whether a community order is imposed, and whether the ban stays at 12 months or extends beyond it.