Quick answer

Ask your pharmacist or doctor before combining. Vitamin K works against warfarin, so starting, stopping or changing a vitamin K2 supplement can make warfarin weaker or stronger, according to NIH and the warfarin label. They describe this for warfarin and similar medicines; for other blood thinners, ask your pharmacist.^1,2

Key facts

Warfarin works by blocking the recycling of vitamin K, which the liver needs to make clotting factors.²
The warfarin label lists increased vitamin K intake as one cause of a weaker response to the medicine.²
MedlinePlus advises eating consistent amounts of vitamin K foods from week to week and talking to your doctor before changing your diet.³
NIH names phenprocoumon, acenocoumarol and tioclomarol, used in some European countries, as medicines similar to warfarin.⁴

Ask your pharmacist or doctor first

Warfarin and similar blood thinners work by blocking vitamin K. A sudden rise in vitamin K from food or supplements can weaken their effect and raise the chance of a blood clot, and a sudden drop can strengthen it and cause dangerous bleeding. NIH and the warfarin label advise keeping vitamin K intake about the same from day to day. NIH names warfarin and similar medicines used in Europe (phenprocoumon, acenocoumarol and tioclomarol), not blood thinners that work in other ways.^1,2,3,4

On the medicine's label

What is documented

NIH says vitamin K can have a serious and potentially dangerous interaction with warfarin and similar blood thinners. A sudden change in how much vitamin K you get, from food or supplements, can cause dangerous bleeding if it drops or blood clots if it rises.^1,4

The warfarin label says the same from the medicine's side. It describes warfarin as a vitamin K antagonist, lists increased vitamin K intake among the things that weaken its effect, and tells people to keep the vitamin K in their diet consistent and to avoid drastic changes.²

Why it happens

Your liver needs vitamin K to finish the clotting factors that let blood clot. Warfarin slows the recycling of vitamin K inside the body, so fewer finished clotting factors are made. Extra vitamin K gives the liver more to work with and pushes back against the medicine, while less vitamin K does the opposite.^2,4

That is why the INR, the blood test your clinic uses to set your warfarin dose, can move when your vitamin K intake changes. The label notes that drugs, changes in diet and other factors all affect it.²

Which blood thinners this covers

NIH describes the interaction for warfarin and similar medicines that work against vitamin K. Other blood thinners work in different ways. The labels for apixaban and rivaroxaban mention vitamin K only to say it is not expected to reverse their effect in a bleeding emergency, and they do not discuss vitamin K supplements.^4,5,6

If you take another kind of blood thinner, such as clopidogrel or daily aspirin, the sources on this page do not address it. That is not a finding either way, so tell your pharmacist about every supplement you take.

Where vitamin K turns up

Vitamin K is in most multivitamins, in vitamin K supplements, in products that combine it with vitamin D, calcium or magnesium, and in some meal replacement shakes and bars. Starting or stopping any of these changes how much vitamin K you get each day.⁴

Leafy green vegetables and certain vegetable oils are the richest food sources. MedlinePlus advises people on warfarin not to eat large amounts of them and to keep the amount steady from week to week.³

Orlistat, a weight-loss medicine, can lower the vitamin K you absorb. Its label asks for close checks of clotting tests in people on warfarin who start it.⁷

What to ask your pharmacist

Does my blood thinner work against vitamin K, like warfarin does?
Do any of my supplements, including my multivitamin, contain vitamin K?
If I want to start or stop a product with vitamin K, how and when should my INR be checked?
Which foods should I keep steady from week to week?

Bring the Supplement Facts panel of everything you take. Vitamin K appears in products you might not think of as a vitamin K supplement.⁴

Questions people ask

Do I have to avoid green vegetables on warfarin?

No. MedlinePlus and the warfarin label advise a normal, balanced diet with a consistent amount of vitamin K, and avoiding sudden big changes, such as starting to eat large amounts of leafy greens. Talk to your doctor before you change your diet.^2,3

Is vitamin K2 different from K1 for warfarin?

NIH's warning covers vitamin K from food and supplements without making an exception for any form, and the warfarin label refers to vitamin K intake in general. Treat K2 products the same way and ask your doctor first.^1,2

What if I already take a vitamin K supplement every day?

Do not stop it suddenly on your own, because a sudden drop in vitamin K can make warfarin's effect stronger. Tell your doctor or pharmacist what you take, so any change can be planned around your INR checks.^1,2

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Sources

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.