This article was last reviewed on
This article waslast modified on 12 July 2023.
What is vitamin K?

Vitamin K is a nutrient that the body requires in small, regular amounts. It is essential for the formation of several substances called coagulation factors that work together to clot the blood when injuries to blood vessels occur. Insufficient vitamin K can lead to excessive bleeding and easy bruising. Vitamin K is also thought to play an important role in the prevention of bone loss. A low concentration of vitamin K within the bloodstream has been associated with low bone density, and there is some evidence that adequate concentrations of vitamin K can improve bone health while reducing the risk of fractures.

There are two natural forms of vitamin K:

  • Vitamin K1 (phylloquinone or phytomenadione) is the natural from of vitamin K that comes from foods, especially green leafy vegetables but also dairy products and vegetable oils. K1 is considered as the "plant form" of vitamin K, but it is also produced commercially to treat some conditions associated with excess bleeding.
  • Vitamin K2 (menaquinones) is made by bacteria, the normal flora in the intestines. Bacteria in the intestines can also convert K1 into K2. Vitamin K2 supplements K1 from the diet to meet the body's requirements.

Since the body is not able to produce a sufficient amount of vitamin K, a certain amount must be taken in through the diet. Adults need around 1 microgram a day of vitamin K for each kilogram of their body weight. It is present in a wide variety of foods, and the normal diet in the U.K. typically supplies enough. Examples of different foods that contain high amounts, often more than the Recommended Daily Allowance (RDA), include leafy green vegetables such as kale, collards, spinach, turnip greens, mustard greens, green leaf lettuce, and other vegetables such as broccoli, green onions, parsley, asparagus, Brussels sprouts, and cabbage. Other sources include dairy products, cereals, vegetable oils, and soybeans. K1 and K2, the types provided by the diet and produced by the body, are both fat-soluble and are stored in the body's fat tissue and in the liver. An adult typically stores about a week's worth of vitamin K.

Interactions with the anticoagulant drug warfarin

People who are on the anticoagulant drug warfarin (COUMADIN®) must be careful of the amount of vitamin K present in the foods that they eat. Vitamin K is an antagonist to warfarin. This means that it counteracts the action of warfarin, reverses its drug activity, and makes it less effective in treating the condition for which it is prescribed. Warfarin is given to people with a variety of conditions such as deep vein thrombosis (DVT) and cardiovascular disease (CVD) to "thin" their blood and prevent inappropriate clotting. The drug works by inhibiting production of vitamin K-dependent clotting factors. Warfarin affects people differently and must be carefully monitored, typically with a prothrombin time (PT) test that gives results as an INR (International Normalised Ratio) value. The INR must be kept within a narrow therapeutic range. Too little warfarin can result in dangerous blood clots, but too much can cause bleeding episodes. Rather than avoid foods rich in vitamin K, it is more important for people on this drug to consume a consistent amount of vitamin K each day. Significant increases or decreases in vitamin K can affect how warfarin works in their bodies.


Accordion Title
About Vitamin K Deficiency
  • Causes

    The most common causes of vitamin K deficiency are insufficient dietary intake, inadequate absorption, and decreased storage of the vitamin due to liver disease, but it may also be caused by decreased production in the intestines.

    • In the U.K., dietary deficiency of vitamin K is rare in healthy individuals but is relatively common in those who are severely ill or who have certain chronic conditions. For example, it is often seen in patients admitted to intensive care units, cancer patients on chemotherapy, chronic dialysis patients, and patients who are at risk for malnutrition, such as those with a poor diet associated with alcohol or drug abuse.
    • Malabsorption, especially impaired absorption of fats due to diseases such as cystic fibrosis, coeliac disease, chronic pancreatitis or Crohns disease, may cause vitamin K deficiency. Cholestatic liver diseases such as a bile duct obstruction or primary biliary cirrhosis can also lead to malabsorption and a deficiency in vitamin K.
    • Some medications, such as antibiotics, antacids, and anti-seizure medications can interfere with the absorption of vitamin K1, decrease the quantity of K2 produced in the intestines, or cause degradation of vitamin K. High doses of aspirin may increase vitamin K requirements.
    • Deficiencies in newborns are associated with haemorrhagic disease of the newborn (also called vitamin K deficiency bleeding or VKDB). This can cause bleeding and bruising and, in severe cases, can lead to fatal bleeding into the brain. VKDB used to be a relatively common occurrence as newborns have small stores of vitamin K when they are born, their intestines do not yet have established normal flora, and breast milk does not provide them with much vitamin K. In addition, if the newborn's mother takes certain drugs during pregnancy, such as anti-seizure medications, then the infant may be vitamin K-deficient at birth. These situations have now been largely resolved with the routine practice of administering a vitamin K injection to all newborns shortly after birth. Intramuscular administration of vitamin K is to be avoided in newborn babies at a high risk of bleeding disorders and other routes of administration need to be considered. When surgery is necessary, the infants may also be given vitamin K before the procedure to prevent excessive bleeding.


  • Signs and Symptoms

    The signs and symptoms associated with vitamin K deficiency may include:

    • Easy bruising
    • Oozing from nose or gums
    • Excessive bleeding from wounds, punctures, and injection or surgical sites
    • Heavy menstrual periods
    • Bleeding from the gastrointestinal (GI) tract
    • Blood in the urine and/or stool
    • Increased prothrombin time (PT)

    In haemorrhagic disease of the newborn, signs and symptoms may be similar to those listed above but, in more serious cases, may also involve bleeding within the skull (intracranial).

    A deficiency of vitamin K may be suspected when symptoms listed above appear in someone who is at an increased risk, such as:

    • Those who have a chronic condition associated with malnutrition or malabsorption
    • Those who have been on long-term treatment with antibiotics; the antibiotics can kill the bacteria that aid in the production of vitamin K2 in the small intestine.
    • Seriously ill patients such as cancer or dialysis patients or those in intensive care units


  • Laboratory Tests

    A deficiency of vitamin K is usually discovered when unexpected or excessive bleeding occurs. In such cases, a prothrombin time (PT) is the main laboratory test performed to investigate the bleeding. If the result is prolonged and is suspected to be due to low levels of vitamin K, then supplementary vitamin K will often be given.. If the bleeding stops and the PT returns to normal, then a vitamin K deficiency is assumed to be the cause.

    Other coagulation tests may occasionally be performed to evaluate someone with symptoms of excessive bleeding and bruising, such as PTT, thrombin time, platelet count, platelet function tests, coagulation factor tests, fibrinogen, von Willebrand factor, and d-dimer.

    Measurements of the vitamin K concentration in blood is rarely used to determine if a deficiency exists. Since this is not a routine test, it is usually performed in a reference laboratory and results may take several weeks.The majority of vitamin K1 is bound to fat particles in the plasma and therefore, results are given as a ratio of vitamin K to triglyceride concentration.

    Increasingly when requesting the vitamin K level, laboratories may offer protein induced by vitamin K absence or antagonist-II (PIVKA-II). This marker increases when the vitamin K is low or inhibited e.g. by warfarin. There are three different subtypes of vitamin K and PIVKA-II therefore provides a sensitive marker of tissue status of vitamin K activity/concentration.

  • Treatment

    Short-term treatment for vitamin K deficiency usually involves either oral supplementation or injections. Long-term or lifetime supplementation may be necessary for those with underlying chronic conditions. The action of vitamin K typically requires 2 to 5 days after it is given to show treatment effect.

    Problems with high levels of natural forms of vitamin K (K1 and K2) have not been reported. These forms have low toxicity, even at high concentrations. However, water-soluble vitamin K3 can be toxic if administered in large quantities. Also, K3 is known to cause haemolytic anemia in infants, so it is not used to treat the very young.

    Vitamin K-dependent clotting factors are produced by the liver. If a person has chronic liver disease, that person may not be able to produce sufficient clotting factors even when adequate vitamin K is available. Vitamin K supplementation may not be effective in those with seriously damaged livers.