Cognitive impairment (CI) develops not only in structural damage to the central nervous system, but also in encephalopathies of dysmetabolic and deficiency etiology. Recently, special attention is focused on the appearance of CI due to the deficiency of cobalamin (vitamin B₁₂) and folic acid (FA), the change in the level of homocysteine (HC). To detect vitamin B₁₂ deficiency is possible by examining key biomarkers in serum based on a decrease in the levels of vitamin B₁₂ and holotranscobalamin, and levels of methylmalonic acid (MMA) and HC. The article presents an analysis of studies conducted in Norway, Korea, India, and other countries to assess the risks of CI in the presence of reduced levels of vitamin B₁₂ in the elderly, which demonstrated a decrease in brain volume in the elderly (according to MRI data) in combination with altered test parameters that assess cognitive functions. In many studies, female patients with reduced levels of vitamin B₁₂ predominated among the studied patients. Also, some studies have demonstrated the effectiveness of complex CI therapy with the inclusion of vitamin B₁₂ (both for oral and intravenous administration). Oral vitamin B₁₂ therapy at a dose of 1000 μg has been shown to be adequate for the treatment of vitamin B₁₂ deficiency. Also, the administration of vitamin B₁₂ for prophylactic purposes is recommended for patients with subnormal or borderline concentrations of vitamin B₁₂ in the blood serum.