Adverse drug reactions (ADRs) account for 6-7 % of all hospital admissions and remain a major clinical problem.Type an ADR is dose-dependent and usually monogenic or oligogenic; an example is warfarin sensitivity, which causes excessive bleeding.A functional promoter polymorphism of vitamin K epoxide reductase complex subunit 1 was found to be associated with inter-individual differences in warfarin dosage and can explain the inter-ethnic differences in warfarin sensitivity.Prospective study of warfarin dosage requirements based on CYP2C9 and VKORC1 genotypes showed that pharmacogenetics-based dosing can improve the time to stable dosing, reduce adverse events and achieve high sensitivity.