Drink-driving law is familiar territory for most operators - limits, breathalysers, bans. Drug-driving law is newer, sharper, and catches people who genuinely believed they were legal. GOV.UK’s guidance sets out two separate offences: it is illegal to drive if you are unfit to do so because of legal or illegal drugs, and it is illegal to drive with certain levels of illegal drugs in your blood even if they have not affected your driving. Read that second limb again. No impairment needs to be proved - the level in your blood is the offence.
The first limb is the old, impairment-based rule: driving while unfit through drugs. “Legal drugs” here means prescription and over-the-counter medicines - the guidance advises anyone taking them and unsure about driving to talk to a doctor, pharmacist or healthcare professional.
The second limb is the 2015-specified-limits regime: set blood limits for named drugs, criminalising driving above the limit regardless of visible impairment. For illegal drugs the limits are set near zero - a zero-tolerance approach, allowing only for accidental exposure. For certain prescribed medicines, higher limits apply, and the law builds in a medical defence.
The police can stop you and administer a field impairment assessment if they think you are on drugs - the guidance describes it as a series of tests, such as walking in a straight line. They can also use a roadside drug screening kit, which tests for cannabis and cocaine from a saliva swab.
If police think you are unfit to drive through drugs, you will be arrested and required to give a blood or urine sample at a police station. A positive evidential test is what leads to charge - and for the specified-limit offences, the lab number alone can be enough.
This is where professional drivers get caught without ever touching an illegal substance. It is an offence to drive with over the specified limits of certain drugs in your blood if you have not been prescribed them - and even prescribed medicines ground the separate impairment offence if they affect your driving.
The guidance lists medicines to discuss with your doctor before driving, including amphetamine-based medicines such as dexamphetamine or selegiline, and clonazepam, among others. The pattern is medicines for pain, anxiety, sleep, and neurological conditions - exactly the prescriptions a 50-year-old driver with a bad back and poor sleep might hold. The safe routine: every time a prescription changes, ask the prescriber or pharmacist one direct question - can I drive commercially on this? - and keep the packaging and the prescription. The medical defence depends on taking the medicine as directed and the driving not being impaired; driving against advice dismantles it.
Over-the-counter products count too. Sedating antihistamines, some cold and flu remedies, and strong painkillers all sit inside “legal drugs” that can make you unfit to drive. The leaflet on the box is part of your compliance chain.
A drug-driving conviction carries, at minimum, a minimum one-year driving ban, an unlimited fine, and up to six months in prison, with the offence staying on the licence for 11 years. For a vocational driver, the ban is the job. But the professional consequences run deeper than the court list.
A drug-driving conviction is precisely the kind of conduct that reaches the traffic commissioner. For drivers, the commissioner can act against the vocational entitlement. For operators, the commissioner guidance on conduct explicitly lists being caught using a mobile phone while driving as the kind of conduct that can trigger a public inquiry - a drug-driving driver in a company vehicle sits in the same territory, raising questions about the operator’s recruitment, supervision and fitness systems. And good repute - the condition every standard licence holder and transport manager must maintain - does not survive careless answers to those questions.
Drug-driving risk belongs in policy, not just in the driving handbook. A written drugs and alcohol policy should cover prescription and over-the-counter medicines, require drivers to declare medications that may affect driving, and explain the specified-limits offence in plain terms - many drivers still believe “prescribed” means “allowed”. Supervisors need a route for handling a driver who appears impaired at the keys, and the courage to use it.
For-hire fleets should also consider where testing fits. Whatever the mechanism, the legal baseline is already there: two offences, roadside screening for cannabis and cocaine, blood or urine at the station, and limits that criminalise levels, not just behaviour.
The law moved drug-driving from “don’t be obviously impaired” to “be under the limit, on whatever you take”. For an industry of professional drivers, that shift deserves a place in every driver’s file - before a roadside swab makes the introduction.