WARFARIN - Warfarin leads to deficiency of vitamin K, which is an important cofactor in certain anticoagulation factor synthesis, resulting in a decrease in production by the liver of factors II, VII, IX, and X, protein C and protein S. The drug is given orally, with the half-life of about 1.5 days, so it takes a few days to take effect and to reverse. The level of anticoagulation can be measured by checking the PT. The international normalized ratio, or INR, is an indirect measure of the PT. Because protein C and protein S lead to systemic anticoagulation, their deficiency can lead to a pro-thrombotic state. Warfarin-induced skin necrosis can occur if a patient deficient in Protein C or Protein S is given warfarin compounds. Alternately, it can occur simply because, due to their short half-lives, protein C and S are the first to decrease with warfarin administration, leading to a pro-thombotic state. For this reason, heparinization should always be performed prior to administering warfarin.