Vitamin K is a fat-soluble vitamin, so your body stores it in fat tissue and the liver. It is best known for its role in helping blood clot, or coagulate, properly. The "K" comes from its German name, Koagulationsvitamin. Vitamin K also plays an important role in bone health.

It is rare to have a vitamin K deficiency. That’s because in addition to being found in leafy green foods, the bacteria in your intestines can make vitamin K. Sometimes taking antibiotics can kill the bacteria and lead to a mild deficiency, mostly in people with low levels to begin with. Vitamin K deficiency can lead to excessive bleeding, which may begin as oozing from the gums or nose. Other things that may lead to vitamin K deficiency include:

• Health problems that can prevent your body from absorbing vitamin K, such as gallbladder or biliary disease, cystic fibrosis, celiac disease, and Crohn's disease
• Liver disease
• Taking blood thinners, such as warfarin (Coumadin)
• Long-term hemodialysis
• Serious burns

Other conditions that benefit from vitamin K include:

Excessive Bleeding

Vitamin K is used to reduce the risk of bleeding in liver disease, conditions where your body doesn’t absorb enough vitamin K, or if you take antibiotics for a long time.

In the U.S., Canada, Great Britain, and many other countries, all newborns receive vitamin K injections to prevent the possibility of bleeding, particularly in the brain. Babies are born without any bacteria in their intestines and do not get enough vitamin K from breast milk to tide them over until their bodies are able to make it.

Even though vitamin K deficiency in newborns is very rare, it is dangerous enough that doctors give the injections. Newborns at greatest risk for vitamin K deficiency are premature or those whose mother had to take seizure medications during pregnancy. Mothers on seizure medications are often given oral vitamin K for 2 weeks before delivery.

Osteoporosis

Your body needs vitamin K to use calcium to build bone. People who have higher levels of vitamin K have greater bone density, while low levels of vitamin K have been found in those with osteoporosis. Similarly, some studies suggest that low levels of vitamin K are associated with a higher risk of osteoarthritis.

There is increasing evidence that vitamin K improves bone health and reduces the risk of bone fractures, particularly in postmenopausal women who are at risk for osteoporosis. In addition, studies of male and female athletes have also found that vitamin K helps with bone health. However, some studies have found that vitamin K didn’t help with bone density.

Foods that contain a significant amount of vitamin K include beef liver, green tea, turnip greens, broccoli, kale, spinach, cabbage, asparagus, and dark green lettuce. Chlorophyll is the substance in plants that gives them their green color and provides vitamin K.

Freezing foods may destroy vitamin K, but heating does not affect it.

There are 3 forms of vitamin K:

• Vitamin K1 or phylloquinone, the natural version of K1 and phytonadione, the synthetic type of K1
• Vitamin K2 or menaquinone
• Vitamin K3 or menaphthone or menadione

Vitamin K1 is the only form available in the U.S. as a supplement. It is available as part of multivitamin complexes or alone, in 5 mg tablets.

Water-soluble chlorophyll is the most common form of vitamin K found over the counter. It is available in tablet, capsule, and liquid forms.

As with all supplements, check with a health care provider before taking vitamin K or giving it to a child.

People whose bodies can’t absorb enough vitamin K, because of gallbladder or biliary disease, cystic fibrosis, celiac disease, or Crohn's disease, will probably get more benefit from a multivitamin containing vitamin K than an individual vitamin K supplement. In certain circumstances, your doctor may give you a vitamin K shot.

The daily Adequate Intake for vitamin K is:

Pediatric

• Infants birth - 6 months: 2 mcg
• Infants 7 - 12 months: 2.5 mcg
• Children 1 - 3 years: 30 mcg
• Children 4 - 8 years: 55 mcg
• Children 9 - 13 years: 60 mcg
• Adolescents 14 - 18 years: 75 mcg

A single injection of vitamin K is also given at birth.

Adult

• Men 19 years and older: 120 mcg
• Women 19 years and older: 90 mcg
• Pregnant and breastfeeding women 14 - 18 years: 75 mcg
• Pregnant and breastfeeding women 19 years and older: 90 mcg

Because of the potential for side effects and interactions with medications, you should take dietary supplements only under the supervision of a knowledgeable health care provider.

At recommended doses, vitamin K has few side effects.

Vitamin K crosses the placenta and is also found in breast milk. Pregnant women and women who are breastfeeding should talk to their doctor before taking vitamin K supplements.

People with a rare metabolic condition called Glucose-6-phosphate dehydrogenase (G6PD) deficiency should avoid vitamin K.

People who take warfarin (Coumadin) should not take vitamin K (see “Possible Interactions”).

People who are receiving dialysis for kidney diseases can have harmful effects from too much vitamin K.

If you are currently being treated with any of the following medications, you should not take vitamin K without first talking to your health care provider.

Antibiotics -- Antibiotics, especially those known as cephalosporins, reduce the absorption of vitamin K in the body. Using them for more than 10 days may lower levels of vitamin K because these drugs kill not only harmful bacteria but also the bacteria that make vitamin K. People who already have low levels of vitamin K, such as those who are malnourished, elderly, or taking warfarin (Coumadin) are at greater risk. Cephalosporins include:

• Cefamandole (Mandol)
• Cefoperazone (Cefobid)
• Cefmetazole (Zefazone)
• Cefotetan (Cefotan)

Phenytoin (Dilantin) -- Phenytoin interferes with the body's ability to use vitamin K. Taking anticonvulsants (such as phenytoin) during pregnancy or while breastfeeding may lower vitamin K in newborns.

Warfarin (Coumadin) -- Vitamin K blocks the effects of the blood-thinning medication warfarin, so that it doesn’t work. You should not take vitamin K, or eat foods containing high amounts of vitamin K, while you are taking warfarin. Speak with your physician for specific dietary guidelines.

Orlistat (Xenical, Alli) and Olestra -- Orlistat, a medication used for weight loss, and olestra, a substance added to some foods, lowers the amount of fat you body can absorb. Because vitamin K is a fat-soluble vitamin, these medications may also lower levels of vitamin K. The Food and Drug Administration now requires that vitamin K and other fat-soluble vitamins (A, D, and E) be added to food products containing olestra. Doctors who prescribe orlistat usually recommend taking a multivitamin with these vitamins. If you should not be taking vitamin K, then you should avoid foods that contain olestra.

Cholesterol-lowering medications -- Bile acid sequestrants, used to reduce cholesterol, reduce how much fat your body absorbs and may also reduce absorption of fat-soluble vitamins. If you take one of these drugs, your doctor may recommend a vitamin K supplement:

• Cholestyramine (Questran)
• Colestipol (Colestid)
• Colsevelam (Welchol)

Booth SL, Al Rajabi A. Determinants of vitamin K status in humans. Vitam Horm. 2008;78:1-22.

Borrelli F, Ernst E. Alternative and complementary therapies for the menopause. Maturitas. 2010 Aug;66(4):333-43. Review.

Breen GA, St. Peter WL. Hypoprothrombinemia associated with cefmetazole. Ann Pharmacother. 1997;31(2):180-184.

Bugel S. Vitamin K and bone health in adult humans. Vitam Horm. 2008;78:393-416.

Crowther MA, Julian J, McCarty D, et al. Treatment of warfarin-associated coagulopathy with oral vitamin K: a randomized controlled trial. Lancet. 2000;356(9241):1551-1553.

Dennehy C, Tsourounis C. A review of select vitamins and minerals used by postmenopausal women. Maturitas. 2010 Aug;66(4):370-80. Review.

Goldman L, Ausiello D. Cecil Medicine, 23rd ed. Philadelphia, PA: Saunders Elsevier. 2007;181.

Kitchin B, Morgan SL. Not just calcium and vitamin D: other nutritional considerations in osteoporosis. Curr Rheumatol Rep. 2007 Apr;9(1):85-92. Review.

McCormick RK. Osteoporosis: integrating biomarkers and other diagnostic correlates into the management of bone fragility. Altern Med Rev. 2007 Jun;12(2):113-45. Review.

Misra D, Booth SL, Tolstykh I, Felson DT, Nevitt MC, Lewis CE, Torner J, Neogi T. Vitamin K deficiency is associated with incident knee osteoarthritis. Am J Med. 2013;126(3):243-8.

Polin: Fetal and Neonatal Physiology, 4th ed. Philadelphia, PA: Saunders Elsevier; 2011.

Shiraki M, Shiraki Y, Aoki C, Miura M. Vitamin K2 (menatetrenone) effectively prevents fractures and sustains lumbar bone mineral density in osteoporosis. J Bone Miner Res. 2000;15(3):515-523.

Thornquist MD, Kristal AR, Patterson RE, et al. Olestra consumption does not predict serum concentrations of carotenoids and fat-soluble vitamins in free-living humans: early results from the sentinel site of the olestra post-marketing surveillance study. J Nutr. 2000;130(7):1711-1718.

Tucker KL. Osteoporosis prevention and nutrition. Curr Osteoporos Rep. 2009 Dec;7(4):111-7. Review.

Whitlock RP, Crowther MA, Warkentin TE, Blackall MH, Farrokhyar F, Teoh KH. Warfarin cessation before cardiopulmonary bypass: lessons learned from a randomized controlled trial of oral vitamin K. Ann Thorac Surg. 2007 Jul;84(1):103-8.

Wilson DC, Rashid M, Durie PR, et al. Treatment of vitamin K deficiency in cystic fibrosis: effectiveness of a daily fat-soluble vitamin combination. J Pediatr. 2001;138(6):851-855.