Poor Metaboliser (PM)

Carries two loss-of-function alleles (e.g., *2/*2, *2/*3, *3/*3). CYP2C19 enzyme activity is absent or severely reduced. CYP2C19-substrate drugs may accumulate, increasing adverse reaction risk. Alternative drugs or significantly reduced doses may be required. For prodrugs like clopidogrel, activation is impaired, leading to therapeutic failure.

Intermediate Metaboliser (IM)

Carries one normal and one reduced-function allele (e.g., *1/*2, *1/*3). Enzyme activity is decreased but not absent. Dose reductions or alternative agents may be considered depending on the medication and clinical context.

Normal (Extensive) Metaboliser (NM/EM)

Carries two normal-function alleles (e.g., *1/*1). Standard drug dosing is appropriate. This is the most common metaboliser phenotype in most populations.

Rapid Metaboliser (RM)

Carries one normal and one gain-of-function allele (e.g., *1/*17). Enzyme activity is increased. Standard dosing is generally appropriate; however, for prodrugs like clopidogrel, enhanced activation may improve efficacy.

Ultra-rapid Metaboliser (UM)

Carries two gain-of-function alleles (e.g., *17/*17). CYP2C19 enzyme activity is significantly elevated. For CYP2C19-substrate drugs, rapid clearance may reduce efficacy at standard doses. For prodrugs like clopidogrel, enhanced activation may increase bleeding risk.