It starts with a swab, not a breathalyser
The roadside drug test that most drivers haven't thought about is a mouth swab. Not a breathalyser. Not a urine sample. A saliva sample collected at the roadside using a DrugWipe device — takes about five minutes to get a result and tests for several drug categories simultaneously.
I watched it happen at the M6 J18 Knutsford check site last summer. DVSA had pulled the truck in for a routine stop. Officer noticed something — not sure what, nothing obvious from the outside — and requested the test. The driver wasn't driving like someone who'd taken anything. But the swab came back positive, and from that point the roadside stop became something else entirely.
He'd had two codeine tablets the night before. Over-the-counter stuff for a bad back. Nothing illegal. And the afternoon got complicated anyway.
What the DrugWipe actually tests for
The roadside saliva test screens for drug groups — cannabis, cocaine, opiates, benzodiazepines, amphetamines, among others. It's a screening tool, not a conclusive test. A positive result at the roadside doesn't mean you're convicted of drug driving. It means the officer has reasonable grounds to require a blood sample.
The blood sample is taken at a police station by a medical professional. That sample is analysed against specific blood concentration limits set out in the Drug Driving (Specified Limits) (England and Wales) Regulations 2014 and section 5A of the Road Traffic Act 1988.
Two categories of drugs sit in those regulations. Illegal drugs with zero-tolerance limits — and prescription drugs with higher but still enforceable limits.
The zero-tolerance limits for illegal drugs
For the drugs the law treats as zero-tolerance, the limits are set just above zero to account for environmental exposure or passive inhalation arguments:
- Cannabis (THC): 2 micrograms per litre of blood
- Cocaine: 10 micrograms per litre
- MDMA (ecstasy): 10 micrograms per litre
- Heroin (6-monoacetylmorphine): 5 micrograms per litre
- Ketamine: 20 micrograms per litre
These are not difficult thresholds to exceed if you've actually taken the drug. Cannabis in particular — 2 micrograms — can still show in blood for drivers who used it days before and consider themselves sober. The detection window in blood is longer than most people assume.
The prescription drug limits — and where it gets complicated
Prescription drugs sit at much higher limits, because at therapeutic doses they're lawful. But there are still limits:
- Diazepam (Valium): 550 micrograms per litre
- Methadone: 500 micrograms per litre
- Morphine: 80 micrograms per litre
- Codeine: 50 micrograms per litre
- Clonazepam: 50 micrograms per litre
- Lorazepam: 100 micrograms per litre
There's a medical defence. If you've taken a prescription medicine according to a doctor's or pharmacist's instructions, and your driving wasn't impaired, you may have a defence. The key word is 'may.' It's a defence you have to argue — it doesn't mean the offence didn't technically occur at the blood test stage.
Codeine is the one that catches people off guard, because you don't need a prescription for it. You can buy co-codamol over the counter at 8mg/500mg strength. Take four tablets — which is what the packaging often allows — over an evening, and your blood codeine level might still be above 50 micrograms per litre the next morning. The DrugWipe might not flag it (it's a screen, not a blood test). But if there's any reason for a blood test and the codeine is above limit, you're in a grey area that requires a solicitor to navigate.
Same issue with prescribed strong opioids. If you're on tramadol or fentanyl patches for pain management, those sit in the opiate class. The fact you have a prescription matters — but you also need to not be impaired. A GP prescribing tramadol to an HGV driver should be discussing fitness to drive at the same time. Not all do.
What happens if the blood test comes back over limit
If your blood test exceeds the specified limit for any drug, you'll be charged under section 5A of the Road Traffic Act 1988. The penalty on conviction: minimum 12-month driving ban, an unlimited fine, up to 6 months in prison, a criminal record. The conviction appears on your licence for 11 years.
For HGV drivers that's career-ending in most cases. A 12-month ban means no work. A criminal record for drug driving means some operators won't touch you after the ban ends. The fine is secondary to the licence consequences.
The offence under section 5A doesn't require you to be impaired. That's the point of the specified limits — if you're over the limit, the offence exists regardless of whether your driving was affected. Different from the older drug driving offence (section 4, 'unfit through drink or drugs'), which required proof of impairment. Section 5A just needs the blood concentration.
The morning-after alcohol problem
Drug testing gets discussed. Alcohol often gets treated as a separate, understood issue. But the morning-after problem for HGV drivers is real and probably more common than the drug conversation.
The legal limit in England, Wales, and Northern Ireland: 35 micrograms of alcohol per 100 millilitres of breath (or 80 milligrams per 100 millilitres of blood). That applies to HGV drivers the same as car drivers — there's no tighter standard for professional drivers in the law, though some operators set their own policies at zero.
The 'one unit per hour' processing rule that gets passed around in pubs is an average, not a guarantee, and it doesn't account for body weight, liver function, what you ate, or how quickly you actually drank. A bigger bloke who put away six pints before midnight might be over the limit at 5am. Might not be. He can't know without testing himself.
Roadside breathalysers for HGV drivers work the same as for car drivers. Any constable can require a test if they have reasonable cause to suspect alcohol. An evidential test at the station follows if the roadside test is positive.
DVSA can refer to police. A DVSA officer at a check site can't administer a breathalyser themselves — but they can radio for police if they've got cause. The two agencies work the same roads.
Prescribed medication and DVLA notification
Some medications require you to notify DVLA as a Group 2 licence holder. This isn't the same as the drug driving offence — it's a separate obligation about whether the condition being treated, or the medication itself, affects your fitness to hold a Group 2 licence.
DVLA publishes guidance on specific medical conditions and medications in a document called 'Assessing Fitness to Drive' — it's on GOV.UK and updated regularly. If a GP prescribes you something in a class that affects alertness or reaction time, ask them explicitly: does this require DVLA notification for a Group 2 holder? Some GPs are good on this. Some aren't.
Failing to notify DVLA when required is a separate offence and can affect your insurance. If you're in an accident while taking undisclosed medication that should have been declared, the insurer will want to know why DVLA wasn't told.
What to do before you swallow anything
Check the packet. Anything containing codeine, diazepam, a strong antihistamine, or an opiate-based painkiller needs a moment's thought before you take it the night before a shift. If in doubt, don't. Or take it after the last drive of the day.
If you're on prescribed medication long-term, talk to your GP about timing your doses relative to driving. A morning dose of some medications might bring your blood level down enough by the time you're on the road. A late-evening dose might still be circulating at start time.
None of this is a reason to avoid necessary medication. It's a reason to plan around it — and to have the conversation with your GP, who should be asking about your occupation when they write the prescription anyway.
If you're tracking your driving hours and shifts in ShiftOwt, you've got a clear record of when your rest periods fall — which is the kind of data that helps you think through medication timing. £5.99 a month for drivers, agency pricing on request.