Quick answer

Vitamin K is needed to make the proteins that let blood clot and that work in bone. Adults need 90 mcg (women) to 120 mcg (men) a day, and most get that from food. K2, the menaquinones, comes from natto, cheese, eggs and meat. If you take warfarin, keep your vitamin K intake steady.^1,2

Key facts

No upper limit is set for vitamin K, because no harm from food or supplements has been reported.²
Natto has about 850 mcg of K2 in 3 ounces, far more K2 than any other food NIH lists.²
MK-7, the K2 form made from natto, stays in the blood much longer than K1.³
The FDA has not authorized a health claim for vitamin K in the US.²

What vitamin K does

Vitamin K is a family of fat-soluble compounds that share one chemical core. Its job is to finish certain proteins so they can work. The best known are clotting factors made in the liver, such as prothrombin, and osteocalcin in bone.²

Vitamin K is also needed to activate matrix Gla protein, found in blood vessel walls, bone and cartilage. What extra vitamin K does in blood vessels is still being studied, and trials so far are few.²

A true shortage of vitamin K is very rare in adults. For a healthy person eating a varied diet, it is almost impossible to get so little that standard clotting tests change.^1,2

Warfarin works by blocking the recycling of vitamin K, which is why vitamin K and warfarin are so closely linked. Anyone on warfarin should read the safety section below before taking any product that contains vitamin K.⁴

What the evidence shows

The first two claims below describe what vitamin K normally does, and are graded moderate as well-established functions of the vitamin. The third is about K2 supplements in particular and rests largely on one trial, so it is graded early. Each shows the amount studied.^2,5

Contributes to normal blood clotting*^1,2,6

Moderate evidence

A long-established role of the vitamin: NIH describes vitamin K as the helper for an enzyme that makes the proteins blood needs to clot, including prothrombin. Most Americans get enough from food, and for a healthy person eating a varied diet it is almost impossible to take in so little that standard clotting tests change. More is not more: ordinary changes in intake have rarely been shown to change clotting time, and in a trial of 42 healthy adults, MK-7 at up to 360 mcg a day did not change thrombin generation, a lab measure of clotting.

Amount studied: Adequate intake of 90 mcg a day for women and 120 mcg for men, from all sources

Contributes to the maintenance of normal bones*^2,7

Moderate evidence

Vitamin K is needed to activate osteocalcin, a protein in bone, and NIH reports that the European Food Safety Authority found a cause and effect relationship between dietary vitamin K and the maintenance of normal bone. Supplements beyond a normal intake are another matter: trials are mixed, a 2019 review of 36 randomized trials found no clear effect on bone mineral density in women after menopause once lower quality trials were set aside, and the FDA has not authorized a health claim for vitamin K in the US.

Amount studied: Adequate intake of 90 to 120 mcg a day from food; supplement trials used from 180 mcg of MK-7 to 45 mg of MK-4 a day

MK-7 may help maintain bone mineral density in women after menopause*^2,5,7

Early evidence

One three-year randomized trial in 244 healthy women after menopause found that 180 mcg of MK-7 a day slowed the usual decline in bone mineral content and density at the lower spine and the femoral neck, though not at the total hip, and bone strength measures held up better than on placebo. It came from a single vitamin K research group, and other trials of vitamin K have found no effect on bone density, so the evidence is early.

Amount studied: 180 mcg of MK-7 a day for three years

K1 and K2

Phylloquinone, or vitamin K1, is the main form in the diet and comes mostly from green leafy vegetables. Phytonadione is the synthetic K1 used in supplements and medicines.^1,2

MK-4 is a short-chain menaquinone. The body makes it from K1 without help from bacteria, and it is found in meat, eggs and dairy foods.²

MK-7 is a long-chain menaquinone made by bacteria. Natto, a dish of fermented soybeans, is very rich in it, and MK-7 supplements are often made from natto.^2,3

Both are well absorbed. MK-7 lasts much longer in the blood and, in a head-to-head study, activated more osteocalcin than K1 did.^2,3

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.²

How much you need

Men need 120 mcg of vitamin K a day from all sources.^1,2

Women need 90 mcg a day, the same during pregnancy and breastfeeding. Pregnant or breastfeeding teens need 75 mcg.^1,2

There is no separate recommended amount for vitamin K2. The adequate intake covers all forms of vitamin K, and it was set from what healthy people eat, because there was not enough evidence to work out a precise requirement.²

US adults average 122 mcg of vitamin K a day from food for women and 138 mcg for men, rising to 164 mcg and 182 mcg when supplements are counted.²

Where vitamin K comes from

Natto, a Japanese dish of fermented soybeans, is the richest food source of K2: 3 ounces has about 850 mcg of MK-7.²

Green leafy vegetables are the richest sources of K1. Half a cup of boiled frozen collards has 530 mcg, a cup of raw spinach 145 mcg, a cup of raw kale 113 mcg and half a cup of boiled broccoli 110 mcg.²

Bacteria in the large bowel make menaquinones, and experts believe these meet at least part of the body's need for vitamin K.^1,2

How much you absorb depends on the source. K1 bound up in green vegetables is taken up far less well than K1 from oils or a tablet, and eating the greens with some fat helps. Limited research suggests long-chain K2 from food may be absorbed better than K1 from green vegetables.²

Side effects, medicines, and who should ask first

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

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.^1,2,4,8

On the medicine's label

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.^1,2,9,10

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.^1,2,11,12

On the medicine's label

Do not start, stop or change a vitamin K supplement, or a multivitamin with vitamin K, if you take warfarin, unless your doctor agrees¹
Ask first if you take any other blood thinner, have had a blood clot, or have a bleeding or clotting disorder
Ask first if you have a condition that limits fat absorption, such as celiac disease, cystic fibrosis or ulcerative colitis, or have had weight-loss surgery²
Ask first if you are pregnant or nursing, or before giving vitamin K to a child

Questions people ask

What does vitamin K2 do for your body?

K2 is one family within vitamin K, and like K1 it activates proteins the body needs for blood clotting and for bone. Most people get enough vitamin K from food. The case for extra K2 rests on early evidence from one bone trial in women after menopause.^2,5

Is K2 better than K1?

Both are absorbed well from supplements. MK-7, a form of K2, stays in the blood longer and, in one head-to-head study, activated more osteocalcin. Whether that makes any difference to health has not been shown.^2,3

What does vitamin K2 do for men?

Its jobs are the same for everyone, but men need more vitamin K: 120 mcg a day against 90 mcg for women. The trial behind the early K2 bone claim studied women after menopause only, and its authors said the results may not apply to men.^1,5

Is vitamin K2 good for your skin?

The sources we use do not test vitamin K2 for skin, so we make no claim about it.²

Should I take vitamin K2 with vitamin D3?

Many products combine them, and both vitamins are absorbed along with dietary fat. No official body sets a ratio between them, and the vitamin D trials we cite did not add K2. If you take warfarin, a D3 and K2 product counts as a vitamin K supplement, so ask your doctor first.^1,2,13

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*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

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