blood has a lower than normal number of red blood cells, due to a deficiency in vitamin B12. The ability to absorb the vitamin declines with age, especially in people over 60 years old. Definition Vitamin B12 is a coordination complex of cobalt, which occupies the center of a corrin ligand and is further bound to a benzimidazole ligand and adenosyl group. It is a deep red solid that dissolves in water to give red solutions. A number of related species are known and these behave similarly, in particular all function as vitamins. This collection of compounds, of which vitamin B12 is one member, are often referred to as "cobalamins". These chemical compounds have a similar molecular structure, each of which shows vitamin activity in a vitamin-deficient biological system, they are referred to as vitamers. The vitamin activity is as a coenzyme, meaning that its presence is required for some enzyme-catalyzed reactions. adenosylcobalamin cyanocobalamin, the adenosyl ligand in vitamin B12 is replaced by cyanide. hydroxocobalamin, the adenosyl ligand in vitamin B12 is replaced by hydroxide. methylcobalamin, the adenosyl ligand in vitamin B12 is replaced by methyl. Cyanocobalamin is a manufactured form of B12. Bacterial fermentation creates AdoB12 and MeB12, which are converted to cyanocobalamin by addition of potassium cyanide in the presence of sodium nitrite and heat. Once consumed, cyanocobalamin is converted to the biologically active AdoB12 and MeB12. The two bioactive forms of vitamin are methylcobalamin in cytosol and adenosylcobalamin in mitochondria. Cyanocobalamin is the most common form used in dietary supplements and food fortification because cyanide stabilizes the molecule against degradation. Methylcobalamin is also offered as a dietary supplement. There is no advantage to the use of adenosylcobalamin or methylcobalamin for treatment of vitamin B12 deficiency. Hydroxocobalamin can be injected intramuscularly to treat vitamin B12 deficiency. It can also be injected intravenously for the purpose of treating cyanide poisoning, as the hydroxyl group is displaced by cyanide, creating a non-toxic cyanocobalamin that is excreted in urine. "Pseudovitamin B12" refers to compounds that are corrinoids with a structure similar to the vitamin but without vitamin activity. Pseudovitamin B12 is the majority corrinoid in spirulina, an algal health food sometimes erroneously claimed as having this vitamin activity. Deficiency Vitamin B12 deficiency can potentially cause severe and irreversible damage, especially to the brain and nervous system. At levels only slightly lower than normal, a range of symptoms such as feeling tired,