A cannabis stop can begin with something as ordinary as a late brake light, drifting within a lane, or a vehicle stopped at the roadside. But how police assess cannabis impairment is more involved than a single test - and a positive roadside saliva swab is not, by itself, proof that someone was driving while impaired.
In the UK, police can investigate suspected drug-driving through observations, a roadside impairment assessment, a saliva screening device and, where appropriate, an evidential blood sample. The route matters because there are two main drug-driving offences with different tests of proof: driving while unfit through drugs, and driving with a specified drug above the legal limit.
For medical cannabis patients in particular, this distinction is crucial. A lawful prescription does not guarantee that driving is lawful in every circumstance, but neither does a THC result automatically mean a conviction.
The two legal routes police use
The first offence is usually described as driving while unfit through drugs. Under section 4 of the Road Traffic Act 1988, the question is whether a person is unfit to drive because of a drug. In practical terms, police must be able to show that the person’s ability to drive was impaired. Cannabis may be the cause, but the prosecution must establish actual unfitness rather than merely the presence of THC.
The second is the specified-limit offence under section 5A of the same Act. For delta-9 THC, the legal limit is two micrograms per litre of blood. This threshold is deliberately low. It is not a measure of whether a person feels high, and it should not be treated as a reliable estimate of their driving ability at the time.
This means a driver can face prosecution under section 5A even where police cannot prove visible impairment, provided an evidential blood test shows THC above the prescribed limit. Conversely, a driver may be prosecuted for being unfit through drugs even if a roadside drug screen is negative or unavailable, if the evidence supports impairment.
How police assess cannabis impairment at the roadside
The process often starts before an officer speaks to the driver. Officers may report signs such as erratic road position, delayed reactions at junctions, unusually slow or fast driving, poor lane discipline, or a collision. None of these behaviours proves cannabis use. Fatigue, illness, prescribed medicines, stress and vehicle faults can all produce similar signs. They are, however, reasons to investigate further.
During a stop, an officer may consider the driver’s speech, coordination, demeanour, eye appearance, response to questions and any smell or visible evidence of cannabis. They may ask whether the driver has taken medication, used cannabis, or consumed other drugs. A driver should take that question seriously, particularly if they use prescribed controlled medicines, but should avoid guessing about exact timings or doses if they do not know them.
Where drug use is suspected, police can require a preliminary drug test or a preliminary impairment test. Refusing without a reasonable excuse is a separate offence and can carry serious consequences.
The preliminary impairment test
A preliminary impairment test is a set of simple roadside tasks used to assess coordination, balance, attention and the ability to follow instructions. It can include walking in a straight line, standing still with eyes closed, or other exercises intended to identify impaired coordination.
These tests are not infallible. A disability, injury, neurological condition, anxiety, poor balance or unsuitable footwear may affect performance. Drivers should state any relevant condition clearly at the time. That does not prevent an arrest, but it creates an important factual record and may affect how the results should be interpreted.
Police may also use a more detailed field impairment assessment, sometimes conducted by a trained drug recognition officer. This can involve observations of pupils, pulse, muscle tone, coordination and behaviour. Such assessments are evidence-gathering tools, not medical diagnoses.
What a cannabis saliva swab can and cannot show
At the roadside, police commonly use an approved oral-fluid device, often referred to as a DrugWipe, to screen for cannabis and cocaine. The device looks for recent drug exposure in saliva. A positive cannabis result can give police grounds to arrest a driver and take them to a police station for further investigation.
It does not establish the concentration of THC in blood. It does not prove the driver was impaired. It is also not the final evidential test used to prove that a driver exceeded the legal THC limit.
A negative saliva result is not necessarily the end of the matter either. If officers believe a driver is unfit through drugs based on driving, behaviour and a preliminary impairment test, they may still pursue the section 4 route. The facts of each stop determine what happens next.
Why blood testing carries the greatest legal weight
Following arrest, police can require an evidential blood sample. That sample is analysed in a laboratory, and the reported concentration is central to a prosecution for driving above the specified limit. In some circumstances, urine may be considered, but blood is the standard evidential sample for drug-driving investigations.
THC is more complicated than alcohol because it does not map neatly on to a single period of impairment. Smoking or vaping cannabis can cause noticeable effects for several hours, while THC may remain detectable in blood for longer, especially after frequent use. Edibles have a slower and less predictable onset, and their effects can last well into the following day.
There is no dependable rule such as waiting a fixed number of hours after cannabis use. Factors including dose, product strength, route of administration, frequency of use, metabolism, food intake and individual physiology can all affect blood levels. For regular users, the low legal threshold creates particular uncertainty.
Police also investigate potential drug-driving after collisions, even where a driver does not initially appear impaired. The legal and insurance consequences can be substantial: a conviction can bring a minimum 12-month driving ban, an unlimited fine, a criminal record and possible imprisonment in the most serious cases.
Medical cannabis: a defence, not a blanket exemption
Patients prescribed cannabis-based medicines may have a statutory medical defence to a section 5A charge. Broadly, it may apply where the medicine was prescribed or supplied for medical or dental purposes and was taken in accordance with the prescriber’s directions or accompanying instructions.
The defence does not apply to driving while unfit through drugs. If a patient’s driving is impaired by their prescribed medicine, they can still be prosecuted under section 4. Nor does a prescription protect someone who has taken more than directed, mixed cannabis with alcohol or other sedating medicines, or used illicit cannabis alongside prescribed treatment.
A prescription label, clinic letter or copy of a prescription can help officers understand the situation, but these documents are not a roadside pass to drive. The central issue remains whether the patient was fit to drive and, if relying on the medical defence, whether they used the medicine as directed.
Patients should discuss driving with their prescriber, particularly when starting treatment, changing a dose, switching from an oil to an inhaled flower product, or experiencing drowsiness, slowed thinking, dizziness or altered coordination. A clinician’s advice is valuable, but the driver remains responsible for deciding not to drive when affected.
CBD products can create their own risk
Over-the-counter CBD products are not the same as prescribed cannabis medicines, and they do not create a medical defence. In the UK, many CBD products are marketed as THC-free or broad-spectrum, but labelling quality varies. Some products may contain trace THC, while others may be inaccurately labelled or contaminated.
A positive drug-driving result after using CBD is unlikely with a reputable product containing no meaningful THC, but the risk is not zero. Consumers who drive should be cautious about products making vague cannabinoid claims, particularly unregulated oils, vapes and edibles bought from unknown sellers.
The safest practical approach is not to treat a saliva test, a prescription, or the absence of obvious intoxication as a guarantee. If cannabis has affected your alertness, coordination or judgement, do not drive. If you are prescribed medical cannabis and depend on a car, keep a clear record of your treatment and have an honest conversation with your prescriber before getting behind the wheel.
