Vitamin K1

Also known as phytonadione, phylloquinone, vitamin k-1.

Last reviewed October 9, 2026 · Jump to references

What is Vitamin K1?

In plain words

Vitamin K1 is a vitamin found mainly in leafy green vegetables and absorbed with fat. The body uses it to make proteins that help blood clot and support bone function. Researchers study its role in bone health, but evidence that supplements prevent fractures is not conclusive. It can interfere with the blood thinner warfarin.

Technical definition

Vitamin K1 (phylloquinone; phytonadione) is a fat-soluble vitamin with a 2-methyl-1,4-naphthoquinone ring and a phytyl side chain, naturally abundant in green plants. Its reduced form serves as a cofactor for gamma-glutamyl carboxylase, enabling activation of vitamin K–dependent proteins involved in blood coagulation and bone metabolism; vitamin K epoxide reductase recycles it. In the United States, vitamin K1 is available in dietary supplements and FDA-approved drug products for specific clotting-related indications.

Educational information only. This page does not provide medical advice or determine whether a supplement is appropriate for you. Review supplements with a qualified healthcare professional, especially if you take medication, are pregnant, have surgery scheduled, or have a chronic medical condition.

Evidence by use

Each use is rated on its own. There is no single overall effectiveness score.