Coumadin (sold under the brand name Warfarin) is an anticoagulant drug that helps prevent blood clots by interfering with the body’s use of vitamin K, a nutrient required for normal clotting. For this reason, people taking warfarin are often advised to avoid foods or supplements that contain vitamin K.
Research published in Blood (November 1, 2004; Volume 104, Issue 9, Pages 2682–2689) examined the effects of vitamin K supplementation in healthy individuals who were stable on warfarin therapy. Participants were given vitamin K₁ supplements beginning at 50 mcg per day for one week, with the dose gradually increased each week. The researchers found that vitamin K intakes below 150 mcg per day did not interfere with anticoagulant therapy. Meals containing foods naturally high in vitamin K also had no measurable effect. When vitamin K intake exceeded 150 mcg per day, the INR decreased. (INR, or International Normalized Ratio, is a standardized measure of blood clotting; a lower INR indicates more rapid clotting.) The authors concluded that eating vitamin K–rich foods such as spinach or broccoli, or taking a multivitamin containing vitamin K, did not interfere with warfarin therapy at typical intake levels.
Additional research published in Blood (March 15, 2007; Volume 109, Issue 6, Pages 2419–2423) found that patients who were unstable on warfarin—meaning their INR values fluctuated widely—were more likely to be deficient in vitamin K. In this study, participants received either a placebo or 150 mcg of vitamin K daily for six months. Those receiving vitamin K experienced less variability in INR values compared with the placebo group, suggesting improved stability of anticoagulation.
People taking warfarin should always discuss dietary changes or supplement use with a doctor and should not attempt to self-medicate.
Newer anticoagulant medications—including rivaroxaban (Xarelto), dabigatran (Pradaxa), apixaban (Eliquis), and edoxaban (Lixiana)—do not work by interfering with vitamin K. As a result, these medications do not carry the same dietary restrictions related to vitamin K intake.
