Many medical and illegal drugs have very serious negative effects on driving ability. In official government statistics, impairment by illegal or medical drugs was recorded as a contributory factor in at least 47 road deaths and 197 serious injuries in Britain in 2014 .
However, the actual figure is likely to be far higher. It's been estimated that 200 deaths a year in the UK may result from drug driving .
In 2005, researchers at the University of Glasgow found one in six (17%) of 1,396 randomly-tested drivers had taken at least one illegal drug in the last 48 hours . UK research in 2000 found evidence of illegal drug use in the bodies of 18% of drivers and 16% of motorcyclists who died in road crashes .
A Brake and Direct Line survey found 3% of UK drivers, the equivalent to one million drivers , admitted driving on illegal drugs at least once in the past year, and one in ten (11%) think they may have been a passenger with a driver on drugs. Three in 10 (29%) admit they wouldn't always speak out to stop a friend driving on drugs .
Effects of illegal drugs
The effects of illegal drugs can be highly unpredictable because they are unregulated. However research has found a number of likely effects on driving, which can be lethal behind the wheel. These are summarised below for some common illegal substances. Some of the effects of drugs on driving can last a long time; in particular, disruption to sleep can make a person risky behind the wheel for days afterwards.
Cannabis: slows reactions; affects concentration; often gives a sedative-like effect, resulting in fatigue; affects co-ordination . Research using driver simulators has found cannabis makes drivers less able to steer accurately and slower to react to another vehicle pulling out .
Cocaine: causes over-confidence; can cause erratic behaviour. After a night out using cocaine, people may feel like they have flu, feel sleepy and lack concentration .
Ecstasy: makes the heart beat faster, which can cause a surge of adrenaline and result in a driver feeling over-confident and taking risks .
Ketamine: can cause muscle paralysis; hallucinations; confusion, agitation, panic attacks; and memory impairment .
LSD: can speed up or slow down time and movement, making the speed of other vehicles difficult to judge; can distort colour, sound and objects; may cause people to see objects which aren’t there; makes people feel panicky and confused .
Speed: makes people feel wide awake and excited, causing erratic behaviour and risk-taking; and can make people panicky. Users have difficulty sleeping, so will be unsafe to drive due to tiredness, sometimes for several days .
Illegal drugs and crash risk
A study of fatal crashes in France between 2001 and 2003 found taking cannabis almost doubles the risk of being involved in a fatal crash . Analysis of road crash hospital admissions in Canada between 2009 and 2011 found cannabis use increases the risk of being involved in a serious crash by four times .
A comprehensive European study into the risks of various drugs found cocaine and illegal opiates raise serious and fatal crash risk by 2-10 times. Drivers taking multiple drugs are 5-30 times more likely to be involved in a severe or fatal crash .
Combining illegal drugs with alcohol is especially deadly: analysis of fatal crashes in the USA found that drivers who have consumed both are 23 times more likely to be involved in a fatal crash than sober drivers .
Many prescription and over-the-counter medications can impair your ability to drive safely, for instance by causing drowsiness or affecting reaction times, coordination, concentration or vision. A UK study in 2000 found 5% of drivers and 4% of motorcyclists who died in road crashes had taken medicines that could have affected their driving . A survey in 2012 found 24% of Australian drivers admit to driving when taking medication that can cause impairment .
Medicines that can impair driving should carry a warning label, but a survey by Brake and Direct Line found one in six (17%) UK drivers admit either ignoring warnings not to drive or not checking the label at all. Almost half (44%) of drivers who use hay fever medication admit sometimes or never checking the instructions to see if it will affect their driving ability .
Brake believes the warnings on medication are often not clear enough. In some cases these warnings tell you not to drive if you feel tired or impaired, when in reality nobody can accurately judge their own impairment. They are also often in small print and therefore hard to spot. In some countries, warning labels are required to be more obvious and give clearer advice on driving.
Effects of medical drugs
Medical drugs that can impair driving include some cough and cold medicines, anti-inflammatories, anti-histamines, antibiotics, antidepressants, epilepsy drugs and sleeping pills. However, many drivers are unaware of this: a Brake and Direct Line survey found three in 10 (30%) are unaware some hay fever and allergy medications can impair driving, almost half (47%) are unaware of the risks of decongestants and six in 10 (60%) don’t know cough medicines can impair driving .
Among hay fever medications, first-generation anti-histamines are well known to cause drowsiness, and some also impair coordination and reaction times in a similar manner to alcohol . However, second- and third-generation antihistamines have also been found to cause drowsiness in some people .
Medical drugs and crash risk
A Norwegian study found the risk of being involved in a road crash doubled or tripled, depending on the type of drug, for up to seven days after being prescribed drugs including opiate painkillers and some tranquilizers .
Research from New Zealand found that drivers who have taken psychoactive illegal or medical drugs (such as some medicines used to treat bipolar disorder) are more than three times more likely to be at fault in collisions than sober drivers .
In the UK, it is an offence to drive while impaired by drugs or when you have certain drugs in your body. The police can stop any driver they suspect of driving on drugs or alcohol, and breathalyse them or test for drugs (see below). They may also test for alcohol or drugs if a driver is stopped for another offence, or if they are involved in a collision. A driver can be charged with driving while impaired by drugs whether the drugs are legal or not.
Drivers convicted of drug driving receive :
- a minimum 12-month driving ban
- a criminal record; and
- a fine of up to £5,000, or up to 6 months in prison, or both.
The police can test suspected drug drivers using a ‘Field Impairment Test’ (FIT), which assesses levels of impairment by drink or drugs. The FIT includes checking the driver’s eyes to see the size of their pupils and how they react to light (because various drugs affect pupil size and reaction in different ways), and tests for balance, coordination and judgement such as standing on one leg while counting out loud . Drivers who fail the FIT can be arrested on suspicion of dangerous driving, and taken to a police station for a blood test to confirm if they’ve taken illegal drugs or medicines that can impair driving.
Whereas previously police had to prove that a drug in a driver’s body was causing impaired driving, resulting in low levels of prosecutions. From 2 March 2015 the law was amended to make it an offence to drive with drugs in your body in England and Wales, removing the need to prove impairment. It is an offence to be over the specified limits for each drug while driving, as it is with drink driving. It is still an offence to drive while impaired by alcohol or drugs, even if below the legal limit, as at present.
Illegal drugs covered by the new rules include cannabis, cocaine, ecstasy and ketamine. Medical drugs covered include diazepam, methadone and morphine. The illegal drug limits are extremely low, effectively zero tolerance . The limits for medical drugs are set at the level where they begin to affect driving, as advised by a panel of medical experts . The penalties for drug driving will remain the same as at present (see above).
Roadside testing devices, similar to breathalysers for detecting alcohol, have been introduced for testing for cocaine and cannabis. These became available to UK police when the new law came into force in March 2015 . They allow the police to test for drugs just as easily as for alcohol, and should enable significantly more drug drivers to be prosecuted.
Learn more: Read the UK government’s facts, advice and resources on drug driving.
 Reported road casualties in Great Britain 2014, Department for Transport, 2015, table RAS50001
 Report of the Review of Drink and Drug Driving Law, Sir Peter North, report to the Department for Transport, 2010
 Drugs in Oral Fluid Part II: Investigation of Drugs in Drivers, University of Glasgow, 2005
 The Incidence of Drugs and Alcohol in Road Accident Fatalities, Transport Research Laboratory, 2000
 Driving licence holding and vehicle availability, Department for Transport, 2014
 Fit to drive: drug driving, Brake and Direct Line, 2014
 A-Z of drugs: Cannabis, Talk to Frank, 2014
 The Influence of Cannabis on Driving, Transport Research Laboratory, 2000
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 A-Z of drugs: Ecstasy, Talk to Frank, 2014
 A-Z of drugs: Ketamine, Talk to Frank, 2014
 A-Z of drugs: LSD, Talk to Frank, 2014
 A-Z of drugs: Speed, Talk to Frank, 2014
 Cannabis intoxication and fatal road crashes in France: population based case-control study, British Medical Journal, 2005
 Cannabis and traffic collision risk, University of Toronto, 2013
 DRUID Final Report: Work performed, main results and recommendations, EU DRUID Programme, 2012
 Drug use and fatal motor vehicle crashes, Columbia University, 2013
 The Incidence of Drugs and Alcohol in Road Accident Fatalities, Transport Research Laboratory, 2000
 Medications and driving: community knowledge, perceptions and experience, Queensland University of Technology, 2012
 Fit to drive: medication and driving, Brake and Direct Line, 2014
 Antihistamines and driving ability: evidence from on-the-road driving studies during normal traffic, University of Utrecht, 2004
 Antihistamines – side effects, NHS Choices, 2013
 Risk of Road Traffic Accidents Associated With the Prescription of Drugs: A Registry-Based Cohort Study, Norwegian Institute of Public Health, 2007
 The culpability of drivers killed in New Zealand road crashes and their use of alcohol and other drugs, New Zealand Environmental Science and Research, 2014
 Drug driving, THINK!, 2014
 Is there a police test for drug driving?, NHS Choices, 2012
 Table of drugs and limits, Department for Transport, 2015
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 Drug driving guidance issued to healthcare professionals, Department for Transport, 2014
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Page last updated: March 2016