The law prohibits driving and other driving-related activities if the person in charge of the vehicle is ‘unfit through drink or drugs’, the definition of unfit is that his or her ability to drive ‘properly’ is impaired. It is not a traffic offence simply to be under the influence of drugs, nor is there currently any drug-driving ‘legal limit’ such as with alcohol.

Driving Behaviour – Was the defendant arrested following an accident, moving traffic offence or erratic driving behaviour? Could unfitness at the time be implied by the driving behaviour? In such cases the evidence of lay or police witnesses as to the driver’s behaviour may be adduced.

In other cases drug-driving charges are brought ‘opportunistically’ as a result of drugs being found in the vehicle, the vehicle smelling e.g. of cannabis, or admissions as to drug use by the defendant during questioning.

Elements of the offence(s)

The quality of prosecution evidence can vary considerably, but in order to succeed a prosecution must show the following three elements in proving the offence:

(a) The driver must be demonstrated to be under the influence of a drug or drugs at the relevant time (via forensic testing of bodily fluids)

(b) The driver must be shown to be impaired and unfit to drive

(c) The impairment must be due to drugs rather than some other cause (e.g. fatigue, shock, physical or mental disability)

Evidence of Intoxication/Being Under the Influence

Forensic evidence as to presence of quantifiable active drug or active metabolite in a blood sample

(a) Presence of inactive metabolites (e.g. THC-COOH also referred to as carboxy-THC or THC-acid for cannabis, Benzoylecgonine for cocaine) does not prove intoxication at the relevant time

(b) Presence of active drug or active metabolites (free THC or active metabolite 11-hydroxy-THC for cannabis, free cocaine etc.) in the sample at quantifiable levels (e.g. THC over 2ng/ml after 3 or more hours) may demonstrate that the defendant was under the influence at the relevant time.

(c) Note that the marker for heroin use is 6-monoacetylmorphine (6-MAM), findings of morphine alone could arise from e.g. codeine-based painkillers. If the prosecution are alleging heroin use it is incumbent on them to test for 6-MAM in the sample.

(d) The half-life in blood of active drugs/metabolites can be short in some cases, therefore a long delay between the incident and a sample being taken can eliminate these substances from the system or greatly reduce their concentrations.

Urine: Urine samples are not normally used because they do not generally allow the time of any intoxication to be determined. However where the defendant refuses a blood test but offers a urine sample the court might in those circumstances regard a positive sample as implying use of the drug around the relevant time.

Saliva? recommended use of saliva samples which offer many of the forensic advantages of blood with the convenience (non-invasiveness) of urine, however there is no legal provision for these at present (although such provisions have been proposed). Taking saliva samples at the time of initial arrest and again contemporaneously with the evidential blood sample could indicate whether a given level represents recent use (falling saliva levels) or residual baseline levels (stable saliva levels) from use in the more distant past. Presence of actual drug residues (e.g. cocaine powder or cannabis fragments) in the mouth may grossly inflate the apparent levels found unless the test is properly conducted by adequately trained personnel.

Evidence of Impairment

Fitness to drive is normally assessed by way of an impairment test, normally conducted at the police station by trained officers and/or a forensic medical examiner. An appropriate standard would be the degree of impairment caused by alcohol at a level equivalent to the legal limit. The court should satisfy itself that the tests were properly conducted and instructions given by the police officer of FME (many FMEs do not have English as their first language) are clear and unambiguous. Ideally such tests should be video-recorded.

Impairment Tests: The five tests are a papillary examination (pupil size and state of eyes), Romberg test (comprehension, time estimation/balance), walk & turn test (comprehension, balance), one-legged stand (balance) and finger to nose test (comprehension, coordination, balance). The performance of ‘normal’ individuals can vary widely, as the balance tests disadvantage older or overweight subjects, hand-eye coordination skills (e.g. finger to nose) is likely to be better in persons involved in skill-sports, and if one never has had the need to estimate time in a number of seconds, the Romberg test performance can be wildly inaccurate.

Many of the tests are highly subjective and can be influenced by prior knowledge on the part of the tester (e.g. that the subject had admitted using drugs, that drugs were seized, or drug use was strongly suspected). It is highly recommended quality-control testing of testers, and that the tester is not made aware of why a subject is to be tested.

Video/CCTV – It is strongly recommended that all such tests be recorded on video or CCTV, preferably with audio, and that all footage of a defendant in custody is demanded and shown to the court, to allow the court to form an objective view as to the condition of the subject. If the police do not have cctv available consider whether a recording could be made yourselves e.g. via mobile phone video camera.

In some cases where CCTV footage has been obtained findings of unfitness have been overruled by the court where the footage has shown the defendant to be acting/speaking normally on admission to custody, or standing steadily for extended periods without swaying, or where demonstrations of the test by officers have been incorrect (e.g. officer taking the wrong number of steps in the walk-and-turn test then failing the defendant for the same reason).

Symptoms Commonly Reported by Police

On arrest, defendants in drug-driving cases are most commonly described by police as having glazed eyes, slurred speech and being unsteady on their feet.

Glazed Eyes – This is an expression frequently reported by police officers describing an individual they suspect to be under the influence of drugs. It is a very vague expression and I still do not understand precisely what it is supposed to mean. Presumably it refers to a dull and unresponsive affect. Different drugs affect the eyes in different ways:

Cannabis – no effect on pupil size but acute intoxication produces a bloodshot appearance from dilating the blood vessels in the sclera (whites of the eyes), can also dry out the tear ducts causing irritation. Can cause jerky eye movements (nystagmus) when following e.g. a finger laterally across the visual field.

Stimulants (amphetamine, cocaine, cathinones) – these drugs tend to dilate pupils, dilated pupils can also be caused by hallucinogens such as LSD, DMT or magic mushrooms.

Opiates (heroin, codeine, morphine, methadone) – these drugs tend to constrict pupils

Depressant drugs (alcohol, barbiturates, tranquillisers, ketamine) – general symptom is drowsiness and reduced responsiveness.

Slurred speech – Different individuals speak in different ways and a report of slurred speech may result from a speech impediment or thick accent in the absence of drugs of any kind.

Depressants – Slurring of speech is generally a symptom of using depressant drugs, most commonly alcohol but also benzodiazepines (e.g. valium), barbiturates (now rarely encountered), ketamine and related drugs, and high-dose opiates.

Stimulants tend to make the user more talkative than usual and increase the rate of speech.

Cannabis has little effect on speech but can lead to the user speaking more slowly, leaving gaps in responses, or diverting off-topic if the user becomes easily distracted.

Unsteady on feet – Again this may reflect organic problems with balance, muscle weakness etc unrelated to drug use, or stiffness after a long period behind the wheel. Also a symptom of using depressant drugs such as alcohol, tranquillisers, hypnotics or high-dose opiates as with slurring of speech.

Stimulants can cause individuals to twitch or appear jumpy or restless.

Cannabis in high doses can cause unsteadiness where this results in a reduction in blood pressure and consequent light headedness.

General comment: All of these descriptors are highly subjective and where a person is suspected of drug-impaired driving in all cases their behaviour or performance on impairment tests (at the roadside or police station) should be video-recorded so the court is able for form an objective view as to the condition of the defendant at the relevant time.

Evidence of Impairment Due to a drug

A finding of unfitness is required before a bodily fluid sample (normally blood) can be required. A doctor (FME) must be satisfied that the defendant has a condition which may be due to a drug.

Note that it usually takes around 15-20 minutes to complete all the impairment tests so if the medical examination is shorter than this the FME may not have had reasonable opportunity to form a view as to whether or not the subject is impaired.

Police officers may be trained to conduct tests and record the results but they do not have the medical training to determine whether a subject is impaired or the reason for any such impairment.

Many FMEs are from overseas and their command of English can vary widely. Where the first language of the FME (or subject) is not English, confusion can arise as to the instructions given causing a subject to fail the test. The command of English of the FME, if appropriate, should be tested in court

Alternative explanations for impairment can include fatigue (particularly for stops in the small hours), organic injury, disability or illness. Accidents can be caused by distractions within the vehicle (e.g. children fighting, a dropped cigarette etc.) unrelated to use of drugs.

Effects of Drugs on Driving

Not all drugs impair driving ability, some drugs may even improve it, other drugs may improve some aspects of driving whilst impairing other aspects. Different individuals vary as to the level of impairment a given dose of a drug might produce. The effects on ability do not tend to be as linear as those found with alcohol, although in general higher doses tend to be more impairing than lower doses.

The main effects of different types of drugs can briefly be summarised as follows:

Cannabis – The effects of cannabis very much depend on the experience of user both of cannabis use and of driving. Cannabis tends to impair tracking ability and judgement in some cases, however drivers under the influence tend to drive more conservatively (slower speeds, greater distances between vehicles, fewer risky overtaking manoeuvres) resulting in little epidemiological evidence of increased accident risk. There is little effect on reaction time, may even reduce reaction time in some cases. The highest increase of risk is for young drivers, particularly females, with little effect in experienced users.

Stimulants (amphetamines/cocaine/cathinones) – stimulants in low doses can increase alertness and improve performance (e.g.. amphetamine pills given to RAF pilots during combat missions), high doses can impair judgement (e.g. making the user ‘jumpy’ and likely to over-react) and lead to more risky/aggressive driving behaviour, excessive self-confidence and feelings of invulnerability, with psychotic reactions in extreme cases of heavy or prolonged use. Comedown effects can lead to excessive fatigue and drowsiness.

Opiates (heroin/codeine/morphine/methadone) – Little effect in experienced users although withdrawal can impair performance. Sedative effects in naïve or occasional users impairing performance. Effect of a moderate dose for an addict would be to make him/her feel ‘normal’.

Depressant Drugs (benzodiazepines and other tranquilisers, barbiturates, ketamine, solvents) – Depressant drugs cause clear and long-established dose-related performance deficits – slower reactions, drowsiness and inattention, impaired judgement – can be disinhibiting like alcohol, with substantially increased accident risk.

Ecstasy – Epidemiological evidence of increased accident risk compared to other stimulants but this may reflect lifestyle issues (e.g. late-night driving) rather than, or compounding, drug effects

Duration of Drug Effects

Note that the effects of different drugs last for different lengths of time, and high doses generally take longer to wear off than low doses. The effects of drugs tend to last longer in naïve users than in experienced users who may have developed tolerance or dependence. The effects of short-acting drugs are short-lived whereas the effects of long-acting drugs may persist for hours or days.

Cannabis – Effects peak around 5 minutes after smoking and subside thereafter within 1-2 hours – Note that the effects of cannabis rarely persist beyond 2 hours and any finding of impairment more than 2 hours after an incident is likely to arise from other causes (or be erroneous).

Stimulants – The effects of stimulants tend to be in two phases, the acute effects of the drug which may improve performance or impair judgement and the rebound depressive effects which tend to cause drowsiness and impair performance.

Cocaine – effects peak very soon after snorting and usually subside within 30 minutes to 1 hour

Amphetamine – effects can last for up to 8 hours depending on the dosage and tolerance of the user

Cathinones – Effects typically last 1-6 hours depending on the drug and dosage used

Tranquillisers – duration of effects depend on the drug and dosage used, can persist for several hours.

Opiates – heroin effects last up to 6 hours, methadone up to 24 hours.

Hallucinogens – effects and duration vary, DMT a few minutes, magic mushrooms typically 4-6 hours, LSD typically 8-12 hours.


For a prosecution to succeed, they need to provide the following evidence

1 – Witness evidence as to reasons for the stop (e.g. police/lay witnesses to erratic driving, accident etc and condition of driver when stopped, why drugs are suspected)

2 – Full details of defendant’s performance on impairment tests (copies of pro-forma e.g. form MG-DDG)

3 – Statement from FME as to findings of impairment and a condition which may be due to a drug

4 – Forensic evidence of presence of active drugs or active metabolites in the blood sample

Solicitors are strongly recommended to demand copies of any CCTV footage of the defendant in custody and/or during performance of impairment tests.

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