Drug Driving Limits UK — What Are They?

The prescribed limits in blood for illegal controlled drugs are:

Cannabis (THC): 2 micrograms per litre of blood — a very low limit set at a level above which impairment is considered likely.
Cocaine: 10 micrograms per litre of blood. Note: the limit applies to cocaine itself; the metabolite benzoylecgonine has a separate limit of 50 micrograms per litre, which can remain detectable for longer.
MDMA (ecstasy): 10 micrograms per litre of blood.
Heroin (morphine): 5 micrograms per litre of blood.
Ketamine: 20 micrograms per litre of blood.
LSD: 1 microgram per litre of blood.
Methamphetamine: 10 micrograms per litre of blood.
6-MAM (a heroin metabolite): 5 micrograms per litre of blood.

The limits for illegal drugs are set deliberately low — essentially a zero-tolerance approach. Even residual levels of cannabis from use the previous day or earlier can exceed the 2 microgram THC limit.

The prescribed limits in blood for controlled medications are set at levels above therapeutic doses — in theory, taking medication at the prescribed dose should not breach the limit. However, some medications have limits that can be exceeded at therapeutic doses in some patients:

Clonazepam: 50 micrograms per litre.
Diazepam: 550 micrograms per litre.
Flunitrazepam: 300 micrograms per litre.
Lorazepam: 100 micrograms per litre.
Methadone: 500 micrograms per litre.
Morphine: 80 micrograms per litre.
Oxazepam: 300 micrograms per litre.
Temazepam: 1,000 micrograms per litre.

For prescribed medications, a statutory medical exception defence is available under section 5A(3) where the drug was taken in accordance with medical instructions and the driver’s driving was not impaired.

Detection times in blood vary significantly between drugs and individuals. Cannabis (THC) may be detectable in blood for up to 12-24 hours after use in occasional users, and longer in regular users. Cocaine and its metabolites may be detectable for 12-48 hours. These are approximate ranges — individual metabolism, body weight, hydration, and frequency of use all affect detection times. The legal question is not whether a drug is detectable but whether it is above the prescribed limit at the time of driving.