Inclusion Criteria1. Patients on long-term warfarin (for prophylaxis of arterial embolism in patients with atrial fibrillation or mechanical heart valve replacement, or secondary prophylaxis after VTE) with a target INR of 2.0-3.0 or 2.5-3.5,
2. Anticoagulant therapy managed by the clinic (HHS - General Hospital) for at least 3 months prior to enrolment, and
3. Maintenance dose of warfarin unchanged for the previous 3 months or longer.
4. The INR result is outside the therapeutic range as follows:
For those with target 2.0-3.0: Either an INR of 1.5-1.9 or an INR of 3.1-4.4. For those with target 2.5-3.5: Either an INR of 1.5-2.4 or an INR of 3.6-4.4.
Exclusion Criteria1. Age <18 years,
2. Long-term (>1 week) change in any other medication
3. Long-term (>1 week) change in diet, especially regarding green vegetables.
4. Attending physician believes the patient is not suitable for the study (e.g. psychiatric disorder, history of non-compliance),
5. Failure to obtain telephone consent.