Police powers // The process // Your rights
Not for no reason at all. But the grounds are much wider than most drivers assume, and one of them is a minor moving traffic offence.
When they can test you
Under the Road Traffic Act, an officer in uniform can require a preliminary drug test at the roadside in three broad situations:
If you are stopped for anything at all, or involved in a collision of any kind, you are within reach of a test. “I hadn’t done anything serious” is not a shield, because it was never meant to be one.
What the swab is
The roadside device is a screening test. That is its legal function. It exists to tell an officer whether there are grounds to arrest you — nothing more.
It does not convict anybody. It is not the evidence of the offence. It does not measure whether you are over the limit, and its result is not what a court decides on.
Only the blood decides. A swab can flag and the evidential sample come back clear — that happened to the founder of this site, on a stop for a broken wing mirror, and it ended in no charge at all. The safeguard is real and it works in both directions.
Refusing
This is the single most expensive misunderstanding on this page, so it gets said plainly.
Failing to provide a specimen for analysis, when driving or attempting to drive, carries obligatory disqualification of at least 12 months, an unlimited fine, and up to 6 months in prison.
In other words: comparable consequences to the offence you were being tested for.
There is a narrow concept of a reasonable excuse — it is genuinely narrow, it is usually medical, and “I didn’t want to” has never been one. If you think you have a medical reason you cannot provide a sample, say so at the time, clearly, and say why.
The logic people apply here is understandable and it is wrong. Refusing does not remove the risk. It replaces one charge with another, and it removes the one thing that might have cleared you — the sample itself.
Failure to provide a specimen for analysis (drive/attempt to drive), Road Traffic Act 1988 s.7(6). Sentencing Council guideline, revised 2017. A conviction for failing to provide also counts as a relevant previous conviction, which raises the minimum disqualification for a later drug or drink driving offence to 3 years.
At the station
If the swab flags, you are arrested and taken to a police station. There, a blood sample is taken by a healthcare professional — a doctor or a nurse, not the arresting officer.
That sample goes to a laboratory. The laboratory result is the evidence. Everything that happens afterwards — charge or no charge — follows from it.
You are entitled to free and independent legal advice at the police station. It is available at any hour, it does not cost you anything, and taking it does not make you look guilty. It is the single most useful thing available to you at that point and most people do not use it.
If it happens to you
This is guidance on how to conduct yourself at the roadside and at the station. It is not legal advice about your case, and nothing on this page reduces any charge. If you are facing one, speak to a motoring solicitor.