Short answer: Vitamin K is not banned, but changes in how much you get matter. Warfarin works by blocking vitamin K's role in clotting, so the NIH Office of Dietary Supplements says people on warfarin need to keep their vitamin K intake consistent. MedlinePlus says to eat steady amounts week to week, avoid large amounts of leafy greens, and talk to your doctor before changing your diet or adding supplements.
The NIH Office of Dietary Supplements (ODS) explains that vitamin K is needed to make proteins involved in blood clotting, including prothrombin (clotting factor II). Warfarin (Coumadin) and some other anticoagulants work against vitamin K, and in turn prothrombin. That is why, ODS says, people taking these anticoagulants need to maintain consistent vitamin K intakes.
More vitamin K than usual can push against warfarin, and less than usual can let warfarin act more strongly. Steady intake keeps the dose and the INR blood test (a measure of how long blood takes to clot) more predictable.
ODS lists green leafy vegetables such as spinach, kale, broccoli and lettuce, vegetable oils, some fruits such as blueberries and figs, and meat, cheese, eggs and soybeans as sources. Vitamin K is also in multivitamins and in supplements that pair it with calcium, magnesium or vitamin D, so a new multivitamin can change daily intake too.
MedlinePlus lists coenzyme Q10, echinacea, garlic, ginkgo, ginseng, goldenseal and St. John's wort among non-prescription or herbal products that may interact with warfarin, along with omeprazole, famotidine, aspirin and NSAID pain relievers such as ibuprofen and naproxen. It says to tell your doctor and pharmacist about all vitamins and supplements, as some may raise bleeding risk.
ODS adds that some medicines, including antibiotics, bile acid sequestrants and orlistat, can lower vitamin K levels. The Xenical (orlistat) label says vitamin K absorption may be decreased and that INR changes have been reported in people on warfarin, so it asks for close monitoring of clotting tests. Testosterone labels also say androgens can change anticoagulant activity in people on warfarin.
Warfarin doses are set from INR results, so it helps to know what changed before a result moved: a new supplement or multivitamin, a week of more or fewer greens, a new antibiotic, or a missed dose. Your prescriber or anticoagulation clinic is the one to say whether a change needs a dose adjustment. Dose Track logs each warfarin dose and any supplement alongside your INR results.
Read more on Dose Track: Vitamin K2 (MK-7) guide · Omega-3 fish oil · Xenical side effects · Blood clots on TRT · Multivitamins