Vitamin K deficiency typically occurs more commonly in infants than adults. But adults with certain health conditions or who take certain medications may have an increased risk.

There are two main kinds of vitamin K. Vitamin K1 (phylloquinone) comes from plants, especially leafy green vegetables like spinach and kale. Vitamin K2 (menaquinone) is naturally created in the intestinal tract and works similarly to K1.

Vitamin K plays an important role in coagulation, better known as blood clotting. Clotting is a process that helps prevent excessive bleeding both inside and outside the body.

Your body needs vitamin K in order to produce the proteins that go to work during the clotting process. If you’re vitamin K deficient, your body doesn’t have enough of these proteins. The telltale sign of vitamin K deficiency is bleeding too much.

Scientists also believe that vitamin K helps bones grow and stay healthy, but they continue to study that relationship.

Vitamin K deficiency is rare in adults because many of the foods we eat contain adequate amounts of K1, and because the body makes K2 on its own. Plus, the body is good at recycling its existing supply of vitamin K. However, certain conditions and some drugs can interfere with vitamin K absorption and creation, making it possible to become deficient.

Vitamin K deficiency is much more common in infants. In infants, the condition is called VKDB, for vitamin K deficiency bleeding.

The main symptom of vitamin K deficiency is excessive bleeding. Keep in mind that bleeding may happen in areas other than at a cut or wound site. The bleeding may also be apparent if someone:

  • bruises easily
  • gets small blood clots underneath their nails
  • bleeds in mucous membranes that line areas inside the body
  • produces stool that looks dark black (almost like tar) and contains some blood

In infants, doctors may observe vitamin K deficiency if there is:

  • bleeding from the area where the umbilical cord is removed
  • bleeding in the skin, nose, the gastrointestinal tract, or other areas
  • bleeding at the penis if the baby has been circumcised
  • sudden bleeding in the brain, which is extremely dangerous and life-threatening

Although vitamin K deficiency is uncommon in adults, certain people are at increased risk if they:

  • take coumarin anticoagulants such as warfarin, which thins the blood
  • are taking antibiotics
  • have a condition that causes the body to not absorb fat properly (fat malabsorption)
  • have a diet that is extremely lacking in vitamin K

Learn more: What is malabsorption syndrome? »

Coumarin anticoagulants interfere with the production of the proteins involved in blood clotting.

Some antibiotics cause the body to produce less of its own vitamin K. Other antibiotics may cause vitamin K to become less effective in the body.

Fat malabsorption leading to vitamin K deficiency may occur in people with:

Keep reading: Small bowel resection »

Newborn infants are at increased risk for vitamin K deficiency for a variety of reasons:

  • breast milk is very low in vitamin K
  • vitamin K does not transfer well from a mother’s placenta to her baby
  • the liver of a newborn infant doesn’t use the vitamin efficiently
  • newborns don’t produce vitamin K2 on their own in the first few days of life

First, your doctor will need to know your medical history to understand if you’re at risk of becoming vitamin K deficient. People at risk are generally those who:

  • take anticoagulants
  • take antibiotics
  • have a condition where fat absorption is a problem

Most likely your doctor will perform coagulation test called the prothrombin time (PT) test to see if a vitamin K deficiency is causing your symptoms. This is a blood test that measures how long it takes for your blood to clot.

A nurse, lab technician, or another healthcare professional trained at drawing blood will take a sample using a small needle. They will then add chemicals to the sample to see how it reacts. Blood usually takes about 11 to 13.5 seconds to clot. If the blood takes longer to clot, your doctor may determine that you are vitamin K deficient.

The lab may also look at the results in a different way, measuring the international normalized ratio (INR). INR is based on a scale that compares results of different laboratories worldwide. A normal INR is about 0.9 to 1.1. For someone taking a blood thinner, it might be about 2 to 3.5. Your doctor will be looking to see if the number is too high.

The treatment for vitamin K is the drug phytonadione, which is vitamin K1. Most of the time doctors prescribe it as an oral medication. A doctor or nurse might also inject it under the skin (as opposed to into a vein or muscle). The dosage for adults ranges from 1 to 25 milligrams (mg).

Doctors will prescribe a smaller phytonadione dose for someone who is taking an anticoagulant. Typically this dosage is about 1 to 10 mg. This is to avoid a complication due to anticoagulants interfering with the body’s vitamin K production.

In infants, the American Academy of Pediatrics recommends that newborns get a single shot of 0.5 to 1 mg vitamin K1 at birth. A higher dose may be necessary if the mother has been taking anticoagulants or anti-seizure drugs.

If left untreated in adults, vitamin K deficiency can result in excessive bleeding and become dangerous. But in almost all cases, vitamin K deficiency is treatable.

In infants where VKDB is identified and treated quickly, the outlook is good. However, if the bleeding, known as an intracranial hemorrhage, lasts too long or goes untreated, brain damage or death can occur.

There is no set amount of vitamin K that you should consume each day. But on an average day, nutritionists consider 120 mcg adequate for men and 90 mcg adequate for women. Some foods, including leafy green vegetables, are extremely high in vitamin K and will give you all you need in one serving.

A single shot of vitamin K at birth can prevent a problem in newborns.

People with conditions involving fat malabsorption should speak to their doctors about taking a vitamin K supplement and having their levels monitored. The same goes for people taking warfarin and similar anticoagulants.