Vitamin K is essential for blood clotting and bone health. K1 is abundant in leafy green vegetables; K2 appears in fermented foods and is partially produced by gut bacteria. Newborns routinely receive vitamin K injections to prevent hemorrhage. People with malabsorption, those taking broad-spectrum antibiotics, or patients on warfarin need careful management of vitamin K intake. High-dose vitamin E can interfere with vitamin K, and synthetic menadione (K3) has known safety concerns; consult a clinician before using supplements.

What Vitamin K does

Vitamin K is a group of fat-soluble compounds best known for enabling blood clotting. In the liver it helps activate clotting factors (through a process called gamma-carboxylation) and supports proteins involved in bone mineralization, such as osteocalcin. Adequate vitamin K reduces the risk of uncontrolled bleeding and helps maintain bone health.

How the body gets vitamin K

There are two main forms: K1 (phylloquinone), found primarily in green leafy vegetables and some plant oils, and K2 (menaquinone), found in fermented foods (natto, some cheeses) and produced by bacteria in the gut. The colonic bacteria can contribute to K2 status, but dietary intake is the main source for most people.

Newborns receive very little vitamin K from the placenta and have sterile guts, so a single injection of vitamin K1 at birth is a standard recommendation in many countries to prevent hemorrhagic disease of the newborn.

Who is at risk for deficiency

People with disorders that impair fat absorption (for example, advanced celiac disease, Crohn's disease, or bile duct obstruction) can develop vitamin K deficiency. Broad-spectrum antibiotics that disrupt gut bacteria may reduce bacterial production of some menaquinones, potentially contributing to low levels. People on long-term anticoagulation with warfarin (coumarin drugs) are not deficient per se but can have their treatment affected by changes in vitamin K intake.

Interactions and safety

  • Warfarin: Vitamin K reduces warfarin's anticoagulant effect. Patients on warfarin should keep vitamin K intake consistent and consult their clinician before making major dietary or supplement changes.
  • High-dose vitamin E can antagonize vitamin K's role in clotting and increase bleeding risk.
  • Antibiotics and some medical conditions can lower gut-derived K2; probiotics may help reestablish flora but evidence is mixed.
Synthetic forms: Menadione (often called vitamin K3) is a synthetic provitamin historically used in some products, but it has safety concerns and is not commonly recommended for routine human supplementation. 1

Food sources and supplementation

Best dietary sources of K1: kale, spinach, collards, Swiss chard, broccoli, and Brussels sprouts. K2 is highest in natto and found in certain fermented cheeses and while gut bacteria produce some menaquinones. Most people get enough vitamin K from a balanced diet. Supplements are available and may be appropriate when recommended by a clinician for specific deficiencies.

When to see a doctor

Seek medical care for unexplained easy bruising, frequent nosebleeds, heavy menstrual bleeding, or prolonged bleeding after injury. If you are starting or stopping antibiotics, changing your diet dramatically, or are on blood thinners, discuss vitamin K with your provider.

Note: some older reports suggest environmental chemicals or preservatives could affect vitamin K production in animals; mechanisms and relevance to humans need careful confirmation. 2

  1. Confirm high-dose vitamin A interferes with vitamin K coagulation activity (evidence and clinical relevance).
  2. Verify studies showing BHT or similar preservatives block vitamin K production in animals and their relevance to humans.
  3. Confirm current regulatory status and clinical guidance on menadione (vitamin K3) use in human supplements and prescriptions.
  4. Confirm specific toxic effects of menadione (e.g., hemolytic anemia and hyperbilirubinemia) and population susceptibility (infants).

FAQs about Vitamin K

How do I get enough vitamin K from my diet?
Eat a variety of green leafy vegetables (kale, spinach, collards), broccoli, and Brussels sprouts for K1. Include fermented foods like natto or certain cheeses for K2 if you eat them. Most people meet needs through food.
Can antibiotics cause vitamin K deficiency?
Broad-spectrum antibiotics can reduce gut bacteria that produce some K2. This may contribute to lower levels in people already at risk, but diet remains the primary source. Talk to your clinician if you have prolonged antibiotic use.
Should people on warfarin avoid vitamin K?
They do not need to avoid vitamin K entirely, but they should keep vitamin K intake consistent and coordinate any diet or supplement changes with their prescribing clinician because vitamin K will alter warfarin's effect.
Are vitamin K supplements safe?
Food forms (K1, K2) are generally safe at dietary levels. High-dose supplements should be used only under medical supervision, especially for people on anticoagulants. Synthetic menadione (K3) has safety concerns and is not routinely recommended.
Why do newborns get a vitamin K shot?
Newborns have low vitamin K stores and sterile intestines, which puts them at risk for bleeding. A single vitamin K1 injection at birth prevents hemorrhagic disease of the newborn and is standard in many countries.