This is an educational list of interactions tied to a public source. It is not a complete medication review and it is not medical advice. Ask a pharmacist, or the clinician who prescribes the medicine, before you start, stop, or change a supplement. A pair that is missing here is not a safety clearance. SupplementScore takes no ads, sponsorships, or affiliate income. How this site is funded.
Warfarin (Coumadin, Jantoven) is sensitive to vitamin K and to several supplements that change clotting or the amount of drug in the blood. This page lists only pairs we can tie to a public source.
This page is about warfarin. It does not cover apixaban (Eliquis), rivaroxaban (Xarelto), or dabigatran (Pradaxa). Vitamin K does not oppose those medicines in the same way.
Vitamin K can make warfarin less effective if the amount suddenly changes
NIH ODS says vitamin K can have a serious interaction with warfarin. Warfarin works by opposing vitamin K. People taking it need a steady vitamin K intake from food and supplements, because a sudden increase can make the blood clot more easily and a sudden decrease can make bleeding more likely. Starting a vitamin K pill, or stopping one you take every day, is a sudden change. Tell the anticoagulation clinic before you do either.
NCCIH says St. John's wort can weaken the effects of many important medicines, and it names warfarin. The usual problem is that the body clears the medicine faster, so there is less of it in the blood. With warfarin, less drug can mean the blood clots more easily. Do not add or stop this herb without the prescriber.
NCCIH says ginkgo may increase the risk of bleeding in people taking anticoagulant drugs such as warfarin, and that it may interact with other drugs. The page does not say every person will bleed. It does say to talk with a clinician before using ginkgo if you take any medicine.
NCCIH says garlic supplements may increase the risk of bleeding. It says this matters especially before surgery and if you take anticoagulants or aspirin. Tell the prescriber before you start a garlic pill. Food amounts of garlic are not the same as a concentrated supplement.
NIH ODS says vitamin E can inhibit platelet aggregation and oppose vitamin K-dependent clotting factors. Large doses taken with an anticoagulant or an antiplatelet medicine, such as warfarin, can increase bleeding risk, especially if vitamin K intake is low. The fact sheet says the supplemental amount needed for a clinically important effect is unknown, but it is probably more than 400 IU a day. Do not start a high dose on your own.
An NIH ODS answer for people taking warfarin says St. John's wort, CoQ10, and green tea extract might make blood thinners such as Coumadin less effective. The sentence is cautious ("might"). It is still a reason to tell the anticoagulation clinic before you add CoQ10.
Fish oil has not clearly raised major bleeding, but INR should still be watched
NIH ODS says high doses of EPA and DHA might increase bleeding time, and that fish oil might raise the INR when taken with warfarin. It also says most research finds that 3 to 6 grams a day of fish oil does not significantly change warfarin control, and a 2014 review concluded that omega-3s do not raise the risk of clinically important bleeding. FDA labeling for prescription omega-3s still says people on anticoagulants should be checked periodically for INR changes. This is a monitoring flag, not an order to stop eating fish.