<P> Warfarin decreases blood clotting by blocking an enzyme called vitamin K epoxide reductase that reactivates vitamin K. Without sufficient active vitamin K, clotting factors II, VII, IX, and X have decreased clotting ability . The anticlotting protein C and protein S are also inhibited but to a lesser degree . A few days are required for full effect to occur and these effects can last for up to five days . </P> <P> Warfarin first came into commercial use in 1948 as a rat poison . In 1954 it was approved for medical use in the United States . It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system . Warfarin is available as a generic medication . The wholesale cost in the developing world is about US $1.12 to 7.20 for a typical month of treatment . In the United States it usually costs less than $25 a month . </P> <P> Warfarin is used to decrease the tendency for thrombosis or as secondary prophylaxis (prevention of further episodes) in those individuals who have already formed a blood clot (thrombus). Warfarin treatment can help prevent formation of future blood clots and help reduce the risk of embolism (migration of a thrombus to a spot where it blocks blood supply to a vital organ). </P> <P> Warfarin is best suited for anticoagulation (clot formation inhibition) in areas of slowly running blood (such as in veins and the pooled blood behind artificial and natural valves) and in blood pooled in dysfunctional cardiac atria . Thus, common clinical indications for warfarin use are atrial fibrillation, the presence of artificial heart valves, deep venous thrombosis, and pulmonary embolism (where the embolized clots first form in veins). Warfarin is also used in antiphospholipid syndrome . It has been used occasionally after heart attacks (myocardial infarctions), but is far less effective at preventing new thromboses in coronary arteries . Prevention of clotting in arteries is usually undertaken with antiplatelet drugs, which act by a different mechanism from warfarin (which normally has no effect on platelet function). </P>

Where does warfarin work in the clotting cascade