Summary: Metformin is recommended as a first-line pharmacological therapy in T2DM and is one of the most frequently prescribed antyhypergycemic drug. Its chronic administration may be associated with vitamin B12 deficiency which in turn may adversely affect the function of the nervous system and worsen polyneuropathy – the most common complication of diabetes. B12 deficiency may result in anemia and may have detrimental influence on central nervous system causing cognitive dysfunction and worsening dementia. Recent studies show that oral supplementation of vitamin B12, if higher doses (1–2 mg daily) are used may be equally effective as intramuscular administration.