Excessive prolongation of the international normalized ratio (INR) occurs frequently in patients taking warfarin; in fact, about one in six INR values is above the desired range. Excessive prolongation of the INR is clinically important because the risk of bleeding approximately doubles for each one point increase in the INR beyond the usual therapeutic range. Thus, treatment strategies which rapidly and reliably lower an excessively prolonged INR into the desired range have the potential to reduce bleeding. When taken by patients with INR values between 4.5 and 10, a small dose of oral vitamin K (1 mg to 2.5mg) reduces the INR into the desired INR range in about 75% of cases within 24 hours of its administration. If warfarin is simply withheld, and no vitamin K is given, about 25% of patients will have an INR in the desired range at 24 hours. However, vitamin K is rarely given to such patients. In a recent survey carried out by our group, less than 20% of such patients would have been given low dose oral vitamin K by a group of physicians who regularly supervise warfarin therapy.