Based on the extensive, poor, intermediate and ultrarapid phenotypes of patients, inferences may be made relative to drug metabolism, ultimately leading to changes in therapeutic drug choice or dosing. Phenoconversion is a phenomenon that occurs when an individual's drug metabolizing capacity is altered due to the combination of a drug-drug interaction and a drug-gene interaction. Phenoconversion can affect pharmacokinetics as well as pharmacodynamics. Some examples of phenoconversions include amiodarone-warfarin, duloxetine-codeine, rifampin-clopidogrel, and rifampinwarfarin. Pharmacists must consider phenoconversion in cases of multi-drug therapy.