Everything You Need to Know About Vitamin K

Vitamin K is fat-soluble vitamin that plays a role in blood clotting in the body. It comes in two forms: vitamin K1 (phylloquinone) and vitamin K2 (menaquinones). Both have a similar chemical structure of 2-methyl-1,4-naphthoquinone.

Vitamin K1 is naturally found in some foods, such as green vegetables like collard greens, spinach, and broccoli. Vitamin K2, one of the main forms of vitamin K, is synthesized by the bacteria found in the intestines.

Vitamin K is also available as a dietary supplement. However, most people get the amount of vitamin K they need through diet alone.

This article reviews the role of vitamin K in the body, its potential uses, side effects, and interactions with medications.

Dietary supplements are not regulated the way drugs are in the United States, meaning the Food and Drug Administration (FDA) does not approve them for safety and effectiveness before products are marketed. When possible, choose a supplement tested by a trusted third party, such as U.S. Pharmacpeia (USP), ConsumerLab.com, or NSF International. 

However, even if supplements are third-party tested, they are not necessarily safe for all or effective in general. Therefore, it is important to talk to your healthcare provider about any supplements you plan to take and ask about potential interactions with other supplements or medications.

Supplement Facts

  • Active ingredient(s): Phylloquinone or menaquinone
  • Alternate name(s): Phylloquinone, menaquinone, menadione, phytonadione
  • Suggested dose: For healthy individuals, follow the recommended adequate intake (AI). Talk to your healthcare provider about other dosing options.
  • Safety considerations: Consult with your healthcare provider before taking vitamin K; it can interact with several medications.
Fresh organic collard greens

Catherine McQueen / Getty Images

Uses of Vitamin K

Supplement use should be individualized and vetted by a healthcare professional, such as a registered dietitian, pharmacist, or healthcare provider. No supplement is intended to treat, cure, or prevent disease.

There is little research to suggest that vitamin K can play a role in disease prevention. The main use of vitamin K is to treat a vitamin K deficiency, which is rare in the United States.

Vitamin K is routinely provided as an intramuscular (IM) injection to newborns to prevent deficiency. The American Academy of Pediatrics recommends that vitamin K be given within the first six hours of life to all newborns.

Vitamin K has also been researched for its role in:

  • Blood clotting
  • Bone health
  • Heart disease prevention

Blood Clotting

Vitamin K helps produce the proteins in the body that are essential for blood clotting. Vitamin K therapy has been used to reverse the effects of anticoagulants (blood thinners).

An example of this may be someone prescribed anticoagulants who suddenly requires emergency surgery. In this case, 2.5 to 5 milligrams (mg) of vitamin K can be administered to reverse the anticoagulation effects and prevent excess bleeding.

Bone Health

Vitamin K is thought to contribute to bone health due to its role in producing proteins responsible for bone formation. However, there is not enough research on vitamin K to show whether vitamin K supplementation plays a direct role in improving bone health or preventing osteoporosis.

Results of research on vitamin K for osteoporosis prevention are mixed and include:

  • Reasearch suggests that vitamin K and calcium combined may improve bone mineral density.
  • One 2017 study showed that higher levels of vitamin K intake may be associated with modest reductions in bone fractures (breaks).
  • Other studies have found no evidence that vitamin K improves bone mineral density or reduces vertebral fractures. An additional study found that vitamin K had no effect on bone mineral density, and the authors concluded that the results did not support a role for vitamin K in osteoporosis prevention.

Heart Disease Prevention

Research on vitamin K supplementation for heart disease prevention has been inconclusive.

Vitamin K has a role in the formation of a protein called matrix Gla-protein (MGP). MGP blocks vascular calcification (mineral deposits in the walls of the arteries and veins), but it depends on vitamin K for activation. Vascular calcification is associated with an increased risk of heart disease.

However, it is still unclear whether supplementing vitamin K can slow vascular calcification.

Furthermore, a review in the Cochrane Database concluded that there is not enough evidence to support the use of vitamin K in the primary prevention of heart disease.

Vitamin K Deficiency

Vitamin K deficiency is rare in adults but happens often in infants.

Newborns are routinely given a standard vitamin K injection right after birth to prevent deficiency. This is because:

  • Newborns do not produce vitamin K right away.
  • Vitamin K from the mother doesn't transfer well to the baby.
  • Breast milk has a low vitamin K content.

In adults, vitamin K deficiency is most often related to malabsorption disorders. People with malabsorption disorders like cystic fibrosis (CF), celiac disease, Crohn's disease, and short bowel syndrome may need supplemental vitamin K.

What Causes a Vitamin K Deficiency?

A vitamin K deficiency can occur from inadequate intake or poor absorption in the gastrointestinal (GI) tract.

Most people get enough vitamin K in their diets. The bacteria in our GI tract also synthesize (combine with) vitamin K.

Poor absorption of vitamin K occurs secondary to a GI condition or malabsorption (difficulty in absorbing or digesting nutrients). Poor absorption may also be a side effect of some medications.

Groups at Risk of a Vitamin K Deficiency

Those at greatest risk of a vitamin K deficiency include:

  • Newborns if they do not receive a standard vitamin K dose after birth and are exclusively breastfed
  • People with GI diseases that result in reduced absorption or fat malabsorption (e.g., celiac disease, Crohn's disease, bowel resections)
  • People with pancreatic insufficiency (when the pancreas does not make enough enzymes to absorb nutrients), such as those with CF
  • People with alcohol use disorder or severe malnutrition

People with a deficiency as a result of fat malabsorption should take an oral supplement of vitamin K in a water-soluble form.

Although rare, there are a few case reports of vitamin K deficiency and complications in people and their infants after experiencing hyperemesis (severe nausea and vomiting) during pregnancy.

How Do I Know If I Have a Vitamin K Deficiency?

Vitamin K status can be evaluated by assessing dietary intake and absorption.

If you have a GI condition or fat malabsorption, you are at greater risk of vitamin K deficiency. Symptoms of malabsorption include frequent diarrhea, light-colored stools, and weight loss.

Vitamin K levels in the blood are not routinely assessed in most people, nor are they a good way to measure for vitamin K.

Prothrombin time is an indicator of vitamin K status. Prothrombin time is a measure of the time it takes blood to clot. In those prescribed anticoagulants, prothrombin times are routinely assessed to monitor the effectiveness of anticoagulant dosing.

Someone with a vitamin K deficiency will also bruise or bleed more easily. Although uncommon, ecchymosis or petechiae (skin conditions) may also be present.

What Are the Side Effects of Vitamin K?

Vitamin K supplements aren't recommended unless your healthcare provider indicates you need additional vitamin K. Side effects of supplementing vitamin K are rare when taken at the recommended dose.

There may be side effects of taking vitamin K and medications such as blood thinners, aspirin, antibiotics, and more. Taking vitamin K with blood thinners can make the medication less effective. Antibiotic use may block the action of and reduce vitamin K status in the body.


Since vitamin K works to clot the blood, those prescribed the blood-thinning medication Jantoven (warfarin) or those with clotting disorders must be mindful of their vitamin K intake.

Adding more vitamin K to your usual intake will make the medications less effective. It is important to keep your vitamin K intake consistent.

Dosage: How Much Vitamin K Should I Take?

Always speak with a healthcare provider before taking a supplement to ensure that the supplement and dosage are appropriate for your needs. 

Most people can meet their vitamin K needs through diet alone. For supplementation, dosing can vary depending on the cause of the deficiency.

At birth, newborns routinely receive a 0.5 to 1 mg vitamin K intramuscular injection to prevent a deficiency.

Adequate Intakes for Vitamin K
 Age and Sex Daily AI (in micrograms)
0–6 months 2
7–12 months 2.5
1–3 years 30
4–8 years 55
9–13 years 60
14–18 years 75
19 years or older 90
Pregnant/nursing females younger than age 19 75
Pregnant/nursing females older than age 19 90
Males 19 and older 120

Note that the AI for vitamin K is provided in micrograms (mcg). Some dosing suggestions for vitamin K are in milligrams.

For conversion, 1 mg equals 1,000 mcg. When looking at supplements, it is important to look at the amounts and units of measure.

What Happens If I Take Too Much Vitamin K?

There are no known adverse events associated with excess vitamin K intake. There is also no tolerable upper limit (TUL) set for vitamin K.

Those who require anticoagulant medications should be mindful of their vitamin K intake since it can reduce the effectiveness of their medication.


Vitamin K interacts with the following medications:

  • Warfarin and other anticoagulants: These drugs work to deplete vitamin-K-dependent clotting factors. Sudden changes in vitamin K intake could alter the anticoagulant effect of the medication. People prescribed warfarin or other anticoagulants are instructed to keep their vitamin K intake consistent.
  • Antibiotics: Antibiotics may kill bacteria in the gut that usually produce vitamin K. Supplements may only be necessary if antibiotics are required over several weeks or they are accompanied by a poor vitamin K intake.
  • Bile acid sequestrants: Bile acid sequestrants include cholestyramine and colestipol. They prevent the reabsorption of bile acids, ultimately reducing cholesterol levels. They may also reduce vitamin K levels, particularly if taken for several years.
  • Orlistat: Orlistat is a medication available both as an over-the-counter (OTC) and prescription product for weight loss. It works by reducing fat absorption, which could, in turn, reduce fat-soluble vitamin absorption. A multivitamin with vitamin K may be suggested in conjunction with orlistat.

How to Store Vitamin K

Follow the manufacturer's guidelines for proper storage. Vitamin K should be stored out of reach of children.

Frequently Asked Questions

  • What foods are good sources of vitamin K?

    Foods with the highest amount of vitamin K are typically leafy green vegetables. Spinach and collard greens are great sources of vitamin K. Other plant-based foods containing vitamin K are asparagus, broccoli, soybeans, legumes, and strawberries. Eggs also provide vitamin K.

  • Should I take vitamin K daily?

    Before taking vitamin K daily, you should talk to your healthcare provider, who can help you weigh the risks and benefits of supplementing vitamin K.

Sources of Vitamin K & What to Look For

Vitamin K is mostly obtained through diet.

It is also available as a single nutrient supplement and in multivitamin products. The amount of vitamin K in multivitamin products is usually less than 75% of the daily value.

Vitamin K can be found in fat-soluble vitamin products, which may also contain vitamins A, D, E, and K.

Food Sources of Vitamin K

Foods with the highest amount of vitamin K are typically leafy green vegetables. Spinach and collard greens are great sources of vitamin K.

Other foods that are a source of vitamin K include:

  • Asparagus
  • Broccoli
  • Soybeans
  • Legumes
  • Canola oil
  • Eggs


Vitamin K is a fat-soluble vitamin that doesn't require supplementation very often. It is easy to get adequate amounts of vitamin K through the diet, and a deficiency is rare. Certain fat malabsorption disorders may put you at higher risk of a deficiency.

Vitamin K functions to help with blood clotting. Therefore, sudden changes in the amount of vitamin K consumed can make anticoagulant medications (e.g., warfarin) less effective.

If you suspect you need to supplement vitamin K, it should be discussed with your healthcare provider first.

19 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Jennifer Lefton, MS, RD/N, CNSC, FAND
Jennifer Lefton, MS, RD/N-AP, CNSC, FAND is a Registered Dietitian/Nutritionist and writer with over 20 years of experience in clinical nutrition. Her experience ranges from counseling cardiac rehabilitation clients to managing the nutrition needs of complex surgical patients.