Vitamin B₁₂, also known as cobalamin or extrinsic factor, is a water-soluble vitamin involved in metabolism. One of eight B vitamins, it serves as a vital cofactor in DNA synthesis and both fatty acid and amino acid metabolism. It plays an essential role in the nervous system by supporting myelin synthesis and is critical for the maturation of red blood cells in the bone marrow. While animals require B₁₂, plants do not, relying instead on alternative enzymatic pathways.

Vitamin B₁₂ is the most chemically complex of all vitamins, and is synthesized exclusively by certain archaea and bacteria. Natural food sources include meat, shellfish, liver, fish, poultry, eggs, and dairy products. It is also added to many breakfast cereals through food fortification and is available in dietary supplement and pharmaceutical forms. Supplements are commonly taken orally but may be administered via intramuscular injection to treat deficiencies.

Vitamin B₁₂ deficiency is prevalent worldwide, particularly among individuals with low or no intake of animal products, such as those following vegan or vegetarian diets, or those with low socioeconomic status. The most common cause in developed countries is impaired absorption due to loss of gastric intrinsic factor (IF), required for absorption. A related cause is reduced stomach acid production with age or from long-term use of proton-pump inhibitors, H₂ blockers, or other antacids.

Deficiency is especially harmful in pregnancy, childhood, and older adults. It can lead to neuropathy, megaloblastic anemia, and pernicious anemia, causing symptoms such as fatigue, paresthesia, cognitive decline, ataxia, and even irreversible nerve damage. In infants, untreated deficiency may result in neurological impairment and anemia. Maternal deficiency increases the risk of miscarriage, neural tube defects, and developmental delays in offspring. Folate levels may modify the presentation of symptoms and disease course.

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. Several related species behave similarly to function as vitamins. This collection of compounds is sometimes 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 having vitamin activity as a coenzyme, meaning that its presence is required for some enzyme-catalyzed reactions.

Cyanocobalamin is a manufactured form of B₁₂. Bacterial fermentation creates AdoB₁₂ and MeB₁₂, which are converted to cyanocobalamin by the 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 B
₁₂ 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 forms for the 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.