Quick answer

NIH says vitamin K has not been shown to cause harm and sets no upper limit. For people taking warfarin, NIH says a sudden rise can cause blood clots and a sudden drop dangerous bleeding. A trial in healthy adults found no change in a clotting lab test. Ask your doctor if you take a blood thinner.^1,2,3

Key facts

NIH's consumer sheet says vitamin K has not been shown to cause any harm, and the expert panel set no upper limit.^1,2
NIH says that for people taking warfarin, a sudden change in vitamin K can cause dangerous bleeding if intake falls or blood clots if it rises.¹
In 42 healthy adults, MK-7 at 10 to 360 mcg a day did not change thrombin generation, a lab measure of how readily blood clots.³
In healthy volunteers kept on acenocoumarol, a warfarin-like blood thinner, MK-7 at 45 mcg a day lowered the INR by about 40%.⁴
NIH says that in healthy people eating a varied diet, getting so little vitamin K that standard clotting measures change is almost impossible.²

What NIH says about vitamin K and clotting

NIH explains that vitamin K works as a helper for the enzyme that makes several proteins the body uses to clot blood. That is why severe shortage causes bruising and bleeding problems. NIH also says vitamin K status is not routinely tested except in people who take anticoagulants or have bleeding disorders, and then by the prothrombin time, which measures how long blood takes to clot.^1,2

The question here runs the other way: can extra vitamin K push the body toward clotting? NIH's answer for most people is that vitamin K has not been shown to cause any harm. The expert panel that sets intakes found no adverse effects from vitamin K in food or supplements in people or animals, which is why no upper limit exists.^1,2

No upper limit has been set for vitamin K, because of its low potential for harm: the expert panel that sets intakes found no reported adverse effects from vitamin K in food or supplements in people or animals. This does not apply to anyone taking warfarin or a similar blood thinner.^1,2

The exception: warfarin and similar blood thinners

NIH says vitamin K can have a serious, potentially dangerous interaction with anticoagulants such as warfarin. These drugs work by opposing vitamin K, so people taking them need to keep their vitamin K intake from food and supplements consistent. NIH's consumer sheet puts it directly: a sudden change can cause dangerous bleeding if a person consumes less, or blood clots if a person consumes more.^1,2

The warfarin label on DailyMed lists increased vitamin K intake as a cause of a weaker response, and tells people to eat a normal, balanced diet with a consistent amount of vitamin K and to avoid drastic changes such as large amounts of leafy greens. MedlinePlus gives the same advice and asks people to tell their doctor about every vitamin and supplement.^5,6

What studies found in healthy people and in people on blood thinners

In healthy adults not taking blood thinners, MK-7 at up to 360 mcg a day did not change thrombin generation, a lab measure of how readily blood clots.³

In 18 healthy volunteers kept on acenocoumarol, a blood thinner that works like warfarin, MK-7 at 45 mcg a day lowered the INR by about 40%, and 10 or 20 mcg a day lowered it enough to matter in at least 40% and 60% of them. The researchers concluded that people on these medicines should avoid MK-7 supplements.⁴

Researchers comparing K1 and MK-7 warned that supplements giving 50 mcg or more of MK-7 a day may interfere with warfarin-type blood thinners in a way that matters clinically.⁷

A 2016 systematic review of 15 studies in 1,838 people on vitamin K antagonists found that low daily vitamin K intake in observational studies went with less stable anticoagulation, but that the role of daily vitamin K supplements was not established.⁸

Other things that change vitamin K

NIH names three groups of medicines that lower vitamin K: antibiotics taken for more than a few weeks, bile acid sequestrants, and orlistat. For people on warfarin, these can matter, and NIH asks anyone taking medicines regularly to discuss vitamin K with their health care providers.^1,2

Who should ask their doctor

Ask your doctor or pharmacist before starting, stopping or changing a vitamin K supplement, or a multivitamin that contains it, if you take warfarin or a similar blood thinner. Ask too if you have a bleeding or clotting disorder or have had a blood clot. The sources checked do not document an interaction with other blood thinners such as apixaban, which is not a finding either way.^1,2

Questions people ask

Does vitamin K cause blood clots?

NIH says vitamin K has not been shown to cause harm, but that in people taking warfarin a sudden rise in intake can cause blood clots. Ask your doctor if you take warfarin or a similar blood thinner.¹

Can vitamin K cause bleeding or bruising?

NIH links bruising and bleeding to severe vitamin K deficiency, not to extra vitamin K. In people on warfarin, a sudden drop in vitamin K can cause dangerous bleeding.¹

Does vitamin K cause jaundice in babies?

The sources checked do not report jaundice from vitamin K in food or supplements. A systematic review of vitamin K in pregnancy rated its evidence on newborn jaundice very low quality. Ask your baby's doctor.⁹

Does MK-7 interfere with blood thinners?

A trial in healthy volunteers on acenocoumarol found that MK-7 at 45 mcg a day lowered the INR by about 40%, and the authors advised people on these medicines to avoid MK-7 supplements. Ask your doctor.⁴

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Sources

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