In 1935, a scientist in Copenhagen observed that newly hatched chickens receiving a diet containing all of the known essential nutrients were developing a hemorrhagic disease. The problem was believed to be related to a decrease in prothrombin, a substance necessary for normal clotting of blood. The Danish scientist named this newly discovered antihemorrhagic factor vitamin K or “Koagulationsvitamin.” Vitamin K refers to a group of three vitamins called the quinones. The K vitamins have a central ring structure with their varying pharmacokinetics being determined by varying side chains. Phytonadione (K1) is the only K vitamin commercially available in the United States. It is available by prescription in either tablet form or injection.