The cobalamine molecule. Its half-life in the blood is only 6 days, but its half-life in the liver is 400 days

Methylcobalamin (shown) is a form of vitamin B₁₂. The dark red crystals form cherry-colored transparent solutions in water

Vitamin B₁₂ (or vitamin B-12) is also called cobalamin. It has a key role in the normal functioning of the brain and nervous system, and in the formation of blood. It is one of the eight B vitamins.

B₁₂ is normally involved in the metabolism of every cell of the human body, especially DNA synthesis and regulation, but also fatty acid and amino acid metabolism.Fungi, plants and animals (including humans) cannot produce vitamin B₁₂. Only bacteria and archaea have enzymes for its synthesis, although animals can convert it to the version they need.