In 2012, government announced a new offence in regard to driving with a specific controlled drug in the body above that drug’s accepted limit. The aim was to reduce expense, effort and time wasted from prosecutions that fail because of difficulties proving a particular drug impaired a driver.

Following a report from a panel of experts and a drug driving consultation the government decided to take:

a zero tolerance approach to 8 drugs most associated with illegal use, with limits set at a level where any claims of accidental exposure can be ruled out

a road safety risk based approach to 8 drugs most associated with medical uses

a separate approach to amphetamine that balances its legitimate use for medical purposes against its abuse

On 2 March 2015 8 generally prescription and 8 illicit drugs were added into new regulations that came into force in England and Wales. Regulations on amphetamine came into force on 14 April 2015.

Table of drugs and limits

‘Illegal’ drugs (‘accidental exposure’ – zero tolerance approach) Threshold limit in microgrammes per litre of blood (µg/L) benzoylecgonine 50µg/L cocaine 10µg/L delta-9-tetrahydrocannabinol (cannabis) 2µg/L ketamine 20µg/L lysergic acid diethylamide 1µg/L methylamphetamine 10µg/L Methylenedioxymethamphetamine (MDMA) 10µg/L 6-monoacetylmorphine (heroin) 5µg/L

‘Medicinal’ drugs (risk based approach) Threshold limit in blood clonazepam 50µg/L diazepam 550µg/L flunitrazepam 300µg/L lorazepam 100µg/L methadone 500µg/L morphine 80µg/L oxazepam 300µg/L temazepam 1,000µg/L

Separate approach (to balance its risk) Threshold limit in blood amphetamine 250µg/L

The government is unable to provide any guidance on what amounts of dosage would equate to being over the specified limits. There are too many variables, such as physical characteristics, where each person will metabolise the drug at different rates. Eating or drinking will also have an effect on the blood concentration.

How will the new laws affect you if you’re taking prescription medicines?

You should continue taking medicine(s) as advised by your doctor or healthcare professional, or according to the patient information leaflet that comes with the medicine.

The new law gives the police powers to test and arrest drivers suspected of driving after taking certain controlled drugs in excess of specified levels. It also provides a medical defence if you’re taking medicine in accordance with instructions from a healthcare professional or an accompanying leaflet, provided you’re not impaired.

If you drive and take prescription medicine, it may be helpful to keep evidence of this with you in case you’re stopped by the police.

Drug drive: am I fit to drive? video

For further information, read about the law on drugs and driving.