Warfarin is a drug that “thins the blood” by interfering with normal blood clotting (anticoagulation). It is given to prevent blood clots in the legs, lungs, heart, and brain. Patients who take warfarin need routine blood tests to see whether their blood is “too thin,” a condition that can cause serious bleeding. The test that best measures warfarin's actions is the international normalized ratio (INR). Patients with too much anticoagulation have high INRs and high risk for bleeding. If the INR is greater than 5.0, warfarin treatment is often stopped for several days, or vitamin K1 is given to counteract the effects of warfarin. However, vitamin K1 may be unnecessary in some patients, and it can cause adverse effects. Deciding which patients need vitamin K1 is not easy because it is difficult to know which patients will have prolonged abnormal INR values without vitamin K1 treatment.