Poor metaboliser
An individual who expresses a mutant metabolic enzyme, often as a result of a point mutation, where the enzyme has lower activity (higher KM, lower Vmax, or some combination of changes in KM and Vmax that reduce substrate turnover) versus one or more drugs. Alternatively, the individual may have an enzyme that is entirely inactive, such that a different enzyme may assume responsibility for the (less efficient) clearance of the drug.
Poor metabolisers may require lower drug doses than extensive metabolisers to achieve a similar steady state concentration with repeated dosing.