Warfarin is the most widely used oral anticoagulant. Since it has a limited therapeutics index, anticoagulant reaction is mostly needed to be controlled in patients taking warfarin. Besides age, gender, medicines taken with comorbidities, the variations in pharmacokinetics and pharmacogenetic genes also affect the dose of warfarin. VKORC1 and CYP2C9 are two genes responsible for warfarin metabolism. Today the importance and clinical effects of warfarin in metabolism are not known sufficiently. The polymorphisms in these genes cause sensitivity and over anticoagulant even hemorrhagic complication in warfarin reaction. Genetic polymorphism research is beneficial especially in patients where international normalized ratio (INR) level cannot be adjusted. The frequency of these genes is also different in different races and populations. This condition causes variety in warfarin reaction. Today the role of genetics is still ignored in adjusting warfarin dose. The contributions of doing genetic analysis for polymorphisms to patient follow ups and expenditure are still contradictive. Multicentric studies about this subject are going on in the USA and Europe.