Summary: Vitamin B12 deficiency is observed in people with diabetes mellitus, especially type 2. The etiopathogenesis in this group is primarily associated with chronic high-dose metformin intake, comorbidities, and advanced age. In people with type 1 diabetes, on the other hand, vitamin B12 deficiency is observed more often in groups with autoimmune gastritis. Severe deficiency leads to increased microvascular complications, such as diabetic polyneuropathy, retinopathy, and, by increasing homocysteine levels, either accelerated cardiovascular disease or accelerated atherogenesis, which may be associated with increased clinical manifestations of cardiovascular complications.