How Were the Drug Driving Limits Set?

One of the difficulties in introducing the Drug Driving law has been to set appropriate limits. The government had various options. Unlike alcohol some of the drugs this law deals with are illegal.  The Government were eager not to give the impression that it was OK to use a small amount of illegal drugs and drive.

The government had 3 options –

Option 1  to set a ‘lowest accidental exposure limit’ for 8 controlled drugs most associated with illegal drug use and road safety risk based limits recommended by an Expert Panel1 for a further 8 controlled drugs.

Option 2 followed the Expert Panel’s recommendations to include 15 controlled drugs in the regulations with corresponding limits all based on a road safety risk approach.

Option 3, proposed a zero tolerance approach (that is, a ‘lowest accidental exposure limit’) for 16 controlled drugs.


The Government opted for option 1


The initial thought was that it should be a zero tolerance (option 3) but the difficulty with that was the possibility of having drugs in your system innocently, for example from passive smoking of cannabis or from taking legal drugs that contain an element of the illegal substance, this was the thinking behind the inclusion of the ‘lowest accidental exposure limit’ in relation to illegal drugs.

The Governments concern over option 2 was that it would send out a message that it was ok to take a small amount of illegal drugs.


Option 1 was not without it’s critics, especially in relation to cannabis. The limit proposed here and later set as the limit was 2µg  per 100ml of blood  The concern was that cannabis can show up in the system up to 24 days after smoking it.

The Government were criticised by Drugscience (Formerly The Independent Scientific Committee on Drugs (ISCD))  in relation to this approach for creating a disparity in the law “It is regrettable therefore that the Government’s chosen approach, Option 1, is to establish a new disparity in the law, where the detection of eight particular controlled drugs in a driver’s body may lead to prosecution even when the driver is not likely to be impaired, whilst for alcohol and some other controlled drugs which are no less dangerous when driving, a rational risk-based approach will be used. This rejection of a purely risk-based approach in favour of an inconsistent values-based approach does not appear to be a legitimate use of the Road Traffic Act. “

Drugscience go on to say that the option chosen by the Government gives mixed messages in that the other options take either a consistent risk  based approach derived from evidence about the drugs or a consistent Zero Tolerance approach.

The option chosen accepts the risk-based approach for some substances which are controlled by the Misuse of Drugs Act but have widespread medical uses (e.g. benzodiazepines, morphine) but takes a zero-tolerance approach to another eight (e.g. cannabis, LSD).

They say that if the Government accepts the experts’ view on some controlled drugs; that specified limits are appropriate as with alcohol, a simultaneous ‘zero tolerance’ approach for 8 controlled drugs is illogical.

There is also the issue of the controlled drugs which happen to have widespread medical uses, e.g. benzodiazepines and morphine, being used for non medical purposes and yet have a risk-based limit. So a person could use a drug such as morphine for no other reason than for recreation and because it also has a medical use they are allowed to have a higher than zero amount in their blood whereas for drugs with no medical use (and cannabis) there is a zero tolerance. It cannot be right that someone using morphine illegally can have a higher limit and drive than someone using a cannabis based drug for help with MS which is not presecribed.

Drugscience go on to say “Morphine and benzodiazepines are mentioned on many hundreds of death certificates each year, whilst LSD, cannabis, ketamine and MDMA are mentioned on between 0 and 30 or so.”

With this option the Government is proposing a contradictory message; that driving with 8 illicit drugs in the body is more serious than driving with alcohol, or any other legal high in the body. This is confusing, not to mention dangerous.

This option will also lead to 3,100 more proceedings than Option 2 (risk based testing), which must logically consist of proceedings against people who have been stopped with amounts of illicit drugs in their bodies below the thresholds of impairment set out by the Expert Panel. It is very difficult to imagine a convincing justification for how these prosecutions are in the public interest, if we agree that it would not be in the public interest to begin prosecuting people who drive with alcohol detectable in their blood, but below the existing threshold for alcohol impairment.


If you face an allegation of dug driving call us on 01623 600645.

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