Literature Search on the Interaction Between Warfarin and Vitamin K

Abstract

The so-called health foods, such as functional foods and dietary supplements (vitamins and minerals), are increasingly being used worldwide. With the widespread use of health foods, the chance of their concomitant use with medications is also increasing. However, there is a paucity of practical and useful information on drug-nutrient interactions for medical professionals. The goal of this study was to construct a comprehensive database on clinical interactions between medications and essential nutrients such as vitamins and minerals, which is practical and useful for medical professionals. We conducted an intensive search for literature concerning clinical interaction between warfarin and vitamin K using the PubMed database. Thirty-two articles on the adverse effects associated with their concomitant use were identified. The majority of these articles reported disturbances of the anticoagulation effect of warfarin by the interaction of dietary supplements and vitamin K-containing foods (such as vegetables, chlorella, and fermented soybean called natto) with warfarin. The disturbances including significant falls or rises in international normalized ratio (INR), were caused by the sudden initiation or cessation of consumption of multi-vitamins or foods containing vitamin K. To avoid this potential interference with the efficacy of warfarin, patients are often given instructions to maintain a constant dietary intake of vitamin K. However, in patients with low vitamin K status, even a slight increase in vitamin K intake may lead to a significant reduction in sensitivity to warfarin. In conclusion, to avoid serious adverse effects, the upper limit of vitamin K intake to maintain an optimal anticoagulation level should be appropriately estimated in patients on warfarin treatment, taking into consideration the baseline vitamin K status.