Consistent daily Vitamin K intake may contribute to the stabilization of INR values in patients with unstable INRs on warfarin therapy. Subjects with unstable INRs will be identified from the existing patient records of the UNC Anticoagulation Clinics. Eligible subjects will sign an informed consent form and will receive a point-of-care (POC) INR monitoring device. They will undergo 9 weeks of home INR monitoring via the POC device (with monthly clinic follow-up). They will subsequently be provided with 500 mcg Vitamin K tablets to be taken once daily. Weekly POC INR monitoring with monthly clinic follow-up will continue for an additional 8 weeks.