Codeine converted by CYP2D6 to morphine.CYP2C9: CYP2C9 Warfarin , phenytoin , and tolbutamide are examples of drugs with a narrow therapeutic index that are metabolized by CYP2C9.Genetic Polymorphisms in Drug Targets and Response to Drug Therapy: Genetic Polymorphisms in Drug Targets and Response to Drug Therapy Genetic polymorphisms occur commonly for drug target proteins,including receptors, enz