Drug Driving back calculations

A not unfamiliar scenario with drink driving is where there is a delay between driving and giving a breath or blood test. This can occur for a number of reasons such as when someone is arrested sometime after driving or are unable to give a breath sample so a blood sample is required and it takes a while for a health care professional to take blood.

By the time the blood or breath sample is taken the driver may be under the limit. The police may believe that the driver would have been over the limit at the important time, that is to say when he was driving. They are able, with the assistance of an expert, do a back calculation to establish what the reading would have been at the time of driving.

It’s likely that the same sort of scenarios will arise with drug driving and the police will want to establish what the reading was at the time of driving.


As simple as that, any delay that allows the driver to go below the limit means that there will be no way to establish that he or she would have been over the limit at the time of driving. 

Whilst the back calculation is relatively easy for alcohol for drugs, because of the many variable in what the body does to drugs, back‐calculation is much more difficult and was considered to be impracticable by the Panel set up by the government to review this.

If you have been charged with a drug driving offence call us for expert and honest advice on 01623 600645

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