Eight patients (aged 45-79 years) receiving warfarin, whose international normalized ratios (INRs) had been fluctuating for reasons that were not clear, received 100 mcg/day of oral vitamin K1. After vitamin K supplementation, INR fluctuations decreased in nearly all patients. A significant decrease was seen in the INR standard deviation (p < 005), and more INRs were within 0.2 units of the target range (57% vs. 32% prior to supplementation).

Comment: Warfarin is a frequently prescribed anticoagulant that reduces the risk of cardiovascular events in certain high-risk groups. Warfarin works by interfering with the action of vitamin K. The main side effect of warfarin is bleeding, which sometimes causes serious problems. Patients receiving warfarin must have their INR (a measure of blood clotting efficiency) checked frequently, to assure that their warfarin dose is therapeutic but not excessive. In some cases, the INR fluctuates substantially, resulting in both undertreatment and overtreatment. It has been shown previously that keeping vitamin K intake relatively stable is a good way to minimize INR fluctuations. The results of the present study suggest that stabilizing vitamin K intake at a higher level, by taking a daily low-dose vitamin K supplement, would be an even more effective way to prevent INR fluctuations. The extra vitamin K might also reduce the risk of osteoporosis and of vascular calcifications. The risk of developing these two conditions is increased by warfarin supplementation, presumably because the drug interferes with the beneficial effects of vitamin K on bone and vascular tissue.

Reese AM, et al. Low-dose vitamin K to augment anticoagulation control. Pharmacotherapy 2005;25:1746-1751.