Vitamin K also occurs in various forms: K1(phylloquinone) and K2 (menaquinone), both of which occur naturally, and K3 (menadione) a synthetic version that is twice as active biologically, and is only administered to people who have difficulty utilizing the natural forms due to conditions such as reduced bile secretion.. In addition to being available dietarily, approximately half of the body's vitamin K needs are met by the biosynthesis of various bacteria in the intestines. Food sources of vitamin K include dark green leafy vegetables, kelp, alfalfa, egg yolk, yogurt, fish liver oils, and legumes, as well as safflower oil and blackstrap molasses.

Vitamin K's primary function in the body is to assist in normal blood clotting, especially in the synthesis of various proteins involved in the coagulation process. Since the body is able to manufacture its own supply of vitamin K, deficiencies are rare, although they can be compounded by impaired intestinal absorption, overuse of antibiotics (which destroy healthy intestinal bacteria), and poor liver function or liver disease. Symptoms of deficiency include abnormal bleeding or hemorrhaging, and miscarriage due to abnormal blood loss.