a | For prodrugs, such as clopidogrel, CYP2C19 genotypes associated with poor or intermediate metabolism lead to lower formation of the active metabolite and can result in treatment failure. b | For active drugs, such as warfarin, CYP2C9 genotypes associated with poor or intermediate metabolism lead to higher concentrations of the active form of the drug and can result in toxicity. CYP2C19, cytochrome P450 2C19; IM, intermediate metabolizer; NM, normal metabolizer; PM, poor metabolizer.