Warfarin is a drug that “thins the blood” by interfering with normal blood clotting. Doctors prescribe warfarin to prevent blood clots in the legs, lungs, heart, and brain. People who take warfarin need frequent blood tests to make sure that the extent of blood thinning is appropriate. If the blood is thinned too much, serious bleeding from the nose or lungs or in the urine and gastrointestinal tract can occur. Doctors use different methods to reverse excessive blood thinning (excessive anticoagulation) from warfarin. If the degree of thinning is high but there is no bleeding, doctors decrease or stop warfarin therapy and watch the patient carefully. They also may give vitamin K to speed the reversal of warfarin's effects. If the patient is bleeding or needs an immediate procedure that could be complicated by bleeding (such as surgery), doctors give blood products (such as fresh-frozen plasma) through the vein to rapidly reverse anticoagulation from warfarin. Sometimes it is difficult to obtain and give the blood products quickly, and blood products have potentially serious side effects. Other ways to rapidly counteract anticoagulation from warfarin are needed.