Vitamin B₁₂ (cobalamin) deficiency is a common condition in the elderly. Contrary to the established point of view, the absence of hematological changes (macrocytic anemia) does not always necessarily exclude this condition. Damage of the nervous system with the development of a complex of neurological and mental disorders is observed even at borderline levels of vitamin in blood. It is known, that vitamin deficiency is important risk factor of dementia. Cobalamin deficiency can directly lead to cognitive damage or accelerate development of dementia due to the other brain pathology — Alzheimer’s disease, vascular dementia. The review provides information on the biological role of cobalamin in the human body, the prevalence of vitamin deficiency, especially in the elderly patients, and methods of its diagnosis and management. Both traditional parenteral therapy regimens and the possibility of oral intake of cobalamin in the course of deficiency correction are discussed.