Influence of vitamin K on anticoagulant therapy depends on vitamin K status and the source and chemical forms of vitamin K

Nutr Rev. 2005 Mar;63(3):91-7. doi: 10.1111/j.1753-4887.2005.tb00126.x.


Warfarin therapy requires close monitoring to avoid excessive bleeding and to maintain the effective therapeutic concentration assessed with the internationalized ratio (INR). High vitamin K intake can decrease the therapeutic effectiveness of warfarin, while poor vitamin K status appears to increase the sensitivity to small changes in vitamin K intake, especially from supplements. Very large amounts of vitamin K from a single meal with vegetables (400 g of vegetables with 700 to 1500 microg of vitamin K1) can measurably change INR, but occasional typical servings (<100 g) would probably have little lasting impact on INR. Warfarin requirements may change in those altering their intake of dark-green vegetables. The 2005 Dietary Guidelines for Americans recommends 3 cups/week of dark-green vegetables, which contain about 100 to 570 microg/serving of vitamin K1. Less well-known sources and chemical forms of vitamin K, such as MK-7 in natto (a fermented Japanese product), also measurably influence INR. Additional research is needed in warfarin-treated patients to fully quantify the interactions among various sources and chemical forms of vitamin K, age, genotype, and other factors.

Publication types

  • Review

MeSH terms

  • Anticoagulants / metabolism
  • Anticoagulants / therapeutic use
  • Antifibrinolytic Agents / administration & dosage
  • Antifibrinolytic Agents / chemistry*
  • Antifibrinolytic Agents / pharmacology*
  • Coumarins / metabolism
  • Coumarins / therapeutic use
  • Dietary Supplements
  • Dose-Response Relationship, Drug
  • Drug Interactions
  • Humans
  • International Normalized Ratio
  • Vitamin K / administration & dosage
  • Vitamin K / chemistry*
  • Vitamin K / pharmacology*
  • Warfarin / metabolism
  • Warfarin / therapeutic use


  • Anticoagulants
  • Antifibrinolytic Agents
  • Coumarins
  • Vitamin K
  • Warfarin