Vitamin B₁₂ (cobalamin) was originally studied in connection with anaemia. However, it transpired that, in addition to the production of blood cells, it is needed for the proper functioning of the nervous system and mental health. Vitamin B₁₂ deficiency can result from insufficient dietary intake, malabsorption, or exposure to nitrous oxide. Vitamin B₁₂ deficiency can be manifested by megaloblastic anaemia, myelopathy (symptoms mainly in the posterior cords), polyneuropathy, dementia, depressive disorders, and productive symptoms. Treatment consists mainly of intramuscular cobalamin supplementation and dietary modification. In some cases, prophylactic action to prevent vitamin B₁₂ deficiency is justified.