Three types of anticoagulant clinics exist. In the first type, patients present to the clinic for blood-drawing, and wait for their INR. Their dose of warfarin is then prescribed based on these results. Within this type of clinic, patients will be identified at the time that their INR prolongation is discovered. Consent for the study will be obtained, and the patient will be provided with the next numbered study kit, from which a single capsule containing either vitamin K or placebo will be administered. In clinics in which a point-of-care device is used for INR determinations the INR will be confirmed with a venous sample before the patient is enrolled. For the purposes of this study, such centres will be called "group 1" sites.