Metformin is a widely used oral antidiabetic biguanide compound. According to the literature, metformin may lower the serum cyanocobalamin levels. We present the case of a 71-old-male treated with metformin for 15 years. When presenting to a periodic checkup, low serum cyanocobalamin levels where found. Laboratory tests showed levels below normal range for hemoglobin (12.7 g/dL) and hematocrit (37.8%). After patient reevaluation, a change in antidiabetic treatment will be considered if metformin will be found the cause of low serum cyanocobalamin levels. Other cases reported in the literature support this hypothesis, justifying the study of the influence of metformin therapy on serum vitamin B12 levels in patients diagnosed with diabetes. The influence of patient age, metformin dosage, duration of treatment and time since diabetes diagnosis on serum levels of vitamin B12 also need to be determined.