Quick answer

For most adults, no harm from vitamin K in food or supplements has been reported, and no upper limit is set. The exception is warfarin and similar blood thinners: in one study, even 10 to 20 mcg of MK-7 a day shifted clotting tests in people on such a medicine. If you take one, ask your doctor first.^1,2,3

Key facts

NIH reports no adverse effects from vitamin K in food or supplements, in people or in animals.¹
Menadione, a synthetic form sometimes called vitamin K3, is no longer used in supplements.¹
In healthy adults, MK-7 at up to 360 mcg a day did not change a lab measure of clotting.⁴
Long antibiotic courses, bile acid sequestrants and orlistat can lower the vitamin K your body gets.²

What the safety record shows

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

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.⁴

Menadione, sometimes called vitamin K3, is a synthetic form that damaged liver cells in laboratory studies in the 1980s and 1990s, so it is no longer used in supplements or fortified foods.¹

That record describes people who are not on blood thinners. It explains why no upper limit exists; it is not a promise that any amount suits everyone.¹

Warfarin and similar blood thinners

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.^1,2,5,6

On the medicine's label

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.³

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

Put together, vitamin K2 is not a casual addition for anyone on warfarin. Even a small daily amount, or a multivitamin or D3 and K2 product that contains it, can move your INR. The warfarin label calls for more frequent INR checks whenever another product is started or stopped, so any change should be planned with your doctor.^2,3,5

Other medicines

Your doctor may want to check on you

Antibiotics can kill the gut bacteria that make vitamin K. Courses longer than a few weeks may lower the amount available to the body, and cephalosporin antibiotics such as cefoperazone might also block vitamin K's action. NIH says vitamin K supplements are usually not needed unless antibiotic use is prolonged and intake is poor.^1,2

Expected from how the two work, not shown in studies

Ask your pharmacist or doctor first

Bile acid sequestrants such as cholestyramine and colestipol can reduce the absorption of vitamin K and the other fat-soluble vitamins, especially over years of use. The cholestyramine label links long-term use to a bleeding tendency caused by low vitamin K, and MedlinePlus lists vitamin K supplements among the products that interact with it.^1,2,8,9

On the medicine's label

Ask your pharmacist how far apart to take them

Orlistat, a weight-loss medicine, can reduce the absorption of vitamin K. Its label says people on warfarin who start orlistat should have their clotting tests watched closely, and advises a daily multivitamin with vitamins A, D, E and K taken at a separate time from orlistat.^1,2,10,11

On the medicine's label

Does vitamin K2 thicken your blood?

Not in people who already get enough vitamin K, as far as the evidence shows. NIH notes that ordinary changes in vitamin K intake have rarely been shown to change clotting time, and the healthy-volunteer trial above found no change in a lab clotting measure at up to 360 mcg of MK-7 a day.^1,4

Those trials were in healthy people. If you have had a blood clot, or have a bleeding or clotting disorder, ask your doctor before taking vitamin K.

Who should ask first

Anyone taking warfarin or a similar blood thinner: do not start, stop or change vitamin K on your own²
Anyone on another blood thinner, such as apixaban, rivaroxaban, clopidogrel or daily aspirin
People with a condition that limits fat absorption, or who have had weight-loss surgery, who may need their vitamin K status checked¹
Anyone pregnant or breastfeeding, or giving vitamin K to a child

Questions people ask

Is it safe to take vitamin K2 daily?

For adults who do not take blood thinners, no harm has been reported: trials gave K2 every day for up to three years, and no upper limit is set. If you take warfarin or a similar medicine, ask your doctor first.^1,12

Can vitamin K2 cause blood clots?

In healthy adults not on blood thinners, trials have not shown it: MK-7 at up to 360 mcg a day did not change a lab clotting measure. In people on warfarin, more vitamin K weakens the medicine, which can raise the chance of a clot.^2,4

Does vitamin K2 affect sleep?

The sources we use do not mention sleep, and NIH reports no adverse effects from vitamin K in food or supplements.¹

See what we make

Every product shows its whole label and what its lab report covers. The store opens soon.

See the Full Measure line

Sources

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