Vitamin K (K from "Koagulations-Vitamin" in German and Scandinavian languages) denotes a group of lipophilic, hydrophobic vitamins that are needed for the posttranslational modification of certain proteins, mostly required for blood coagulation.

Recently vitamin K has also been lauded for its potential role in the increase of bone mass. Studies have proved that supplemental vitamin K promotes osteotrophic processes and slows osteoclastic processes via calcium bonding. In Japan, a form of vitamin K2 is recognized as a treatment for osteoporosis. However the long term effects and benefits are unknown and it remains controversial.

Vitamin K is used to reduce the risk of bleeding in liver disease, malabsorption syndromes as described earlier, or in association with long-term use of antibiotics. Vitamin K has been used in the treatment of heavy menstrual bleeding. Use of vitamin K for liver disease, although often attempted, is not generally successful. This is particularly true in the late stages of the disease, because at that point the liver is unable to make the blood clotting factors, no matter how much vitamin K is present.