Oral vitamin K1: an option to reduce warfarin's activity.

To report 6 patients taking oral vitamin K1 (phytonadione) to reduce warfarin's activity. Six patient cases are summarized in which oral vitamin K1 was used to reduce the international normalized ratio (INR) in patients at risk of bleeding. The use of oral vitamin K1 to antagonize warfarin's effects is discussed, as well as the benefits of oral vitamin K1 administration and the disadvantages of parenteral vitamin K1 administration. In addition, an extensive literature review of the discovery and clinical development of warfarin and vitamin K1 is described. In patients receiving warfarin therapy who have an increased INR and are at risk of bleeding, oral vitamin K1 therapy may be safer, less painful, and more cost-effective than the traditional parenteral route of administration.