Vitamin B12 is a water-soluble vitamin. Water-soluble vitamins dissolve in water. After the body uses these vitamins, leftover amounts leave the body through the urine.
Typically, water-soluble vitamins can not be stored by the body. Vitamin B12 is special, because the body can store it for years in the liver. Because of this, a vitamin B12 deficience is very rare.
Vitamin B12, like the other B vitamins, is important for metabolism. It helps in the formation of red blood cells and in the maintenance of the central nervous system.
Goldman L, Ausiello D. Cecil Textbook of Medicine. 22nd ed. Philadelphia, Pa: WB Saunders; 2004.
Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes: Thiamin, riboflavin, niacin, vitamin B6, folate, vitamin B12, pantothenic acid, biotin, and choline. Washington, DC: National Academy Press; 1998.