Where no offence-specific guideline exists, refer to General guideline: overarching principles
The limits for illegal drugs are set in line with a zero tolerance approach but ruling out accidental exposure. The limits for drugs that may be medically prescribed are set in line with a road safety risk-based approach, at levels above the normal concentrations found with therapeutic use. This is different from the approach taken when setting the limit for alcohol, where the limit was set at a level where the effect of the alcohol would be expected to have impaired a person’s driving ability.
The analysis of drugs in blood is more complex than that for alcohol and there is a larger margin of uncertainty in the measurements. Concentrations of specified substances in blood for the purposes of this offence are expressed in terms of ‘not less than’ which takes account of the margin of uncertainty for the particular substance.