Drug driving

Doctors and other prescribers now have a legal duty to advise patients on the legal implications of driving whilst impaired through the use of prescription drugs.

  • Date: 31 July 2014

DOCTORS and other prescribers have an obvious professional obligation to provide clinical advice to patients on the risks associated with certain medicines, such as serious and common side-effects. This will include whether particular drugs may cause sleepiness or otherwise impair driving.

Legal responsibility for deciding whether the ability to drive might be impaired on any given occasion still rests with the individual taking a drug. Section 4 of the Road Traffic Act 1988 makes it an offence to drive whilst impaired through drugs whether for legitimate medical purposes or not. However, the law in this area is about to become somewhat more complicated.

In order to bring enforcement more in line with that for drink driving a new offence of driving with certain controlled drugs in excess of specified levels in the body is expected to come into force on 2 March 2015. This will operate in addition to the existing law on drug-impaired driving. But the new law will set out a statutory "medical defence" for patients taking medicines in accordance with instructions.

Roadside drug screening devices are being developed that will allow police to test saliva to identify if a driver has taken one or more controlled drugs on a specific list. An individual testing positive may then be requested to provide a blood sample to test if that drug is present above a specified limit. The list of controlled drugs falls into two broad groups: so-called "zero tolerance" or commonly abused drugs, such as cannabis or heroin, and mainly licensed medicines though still with significant liability to be abused, such as benzodiazepines and methadone.

Patients found to have a specified drug present in their blood above the legal limit will generally be entitled to raise the statutory "medical defence" if the drug was "lawfully prescribed, supplied or purchased over-the-counter, for medical or dental purposes and was taken in accordance with advice given by the person who prescribed or supplied the drug, and in accordance with any accompanying written instructions (so far as the latter are consistent with any advice of the prescriber)".

Limits on "zero tolerance" drugs will be set low – though a small minority of patients prescribed a specific agent may still be able to claim a "medical defence". Any individual taking a controlled drug from either group in excess of the specified limits and without a "medical defence" can be prosecuted under the Road Traffic Act.

A patient will be entitled to raise the statutory "medical defence" at any point in their dealing with police and might then not be asked to provide a blood sample. Doctors or prescribers should advise patients to keep some suitable documentation with them when they are driving that provides evidence they are taking a controlled drug under prescription or in accordance with a leaflet accompanying the medicine.

Again, this "medical defence" will apply only provided that driving ability is not impaired.

Department of Transport guidance on drug driving offers additional advice for prescribers and suppliers of controlled drugs on how to advise patients either in discussion or in the development of practice leaflets. The guidance also summarises existing best practice on providing general advice to patients in regard to “medicines and driving”. Key points include:


  • Do not to drive if any symptoms or signs develop suggesting driving may be impaired, such as experiencing sleepiness, poor coordination, impaired or slowed thinking, dizziness or visual problems.
  • Do not drive at certain times when the risk may be temporarily increased, for example, when first starting or when first increasing or reducing the dose of a medicine that may potentially impair driving.
  • Be aware that alcohol taken in combination with other impairing drugs can substantially increase the risk of accidents.

The guidance also points out particular circumstances in which the risk of impairment whilst taking a medicine is increased and thus driving should be avoided. Such situations could include:

  • if another prescribed medicine is added that could also impair driving alongside the already potentially impairing medicine(s)
  • taking an over-the-counter medicine that could also potentially impair driving if taken alongside the prescribed medicine
  • if other medicines are being obtained from other prescribers that may have an impact on driving
  • if there is a developing medical condition that could increase the risk of the impairing side-effects from the prescribed medicine (for example, during the development of a serious illness with recent marked loss of weight)
  • if the patient takes any new medicines that are known to be able to affect the metabolism of an existing medicine and so might impair driving
  • other relevant situations, such as the effects of age or the re-initiation of a medicine that previously caused a period of sleepiness that impaired driving.


ACTION Be aware of your professional obligation to advise patients on the legal implications of driving whilst impaired through the use of prescription drugs. Inform patients on the changes to the law coming next year.

This page was correct at the time of publication. Any guidance is intended as general guidance for members only. If you are a member and need specific advice relating to your own circumstances, please contact one of our advisers.

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