Vitamin B12 Levels in Patients with Type 2 Diabetes Mellitus on Metformin.

BACKGROUND Due to the clinical benefits of metformin, its associated side effects such as vitamin B12 deficiency are usually overlooked and rarely investigated. OBJECTIVE This study was carried out to determine the serum level of vitamin B12 in Nigerian patients with type 2 diabetes mellitus (T2DM) on metformin. METHODS Serum vitamin B12 level was determined using high performance liquid chromatography (HPLC) in 81 T2DM patients who have been on metformin for 5 years or more. Vitamin B12 deficiency was defined as serum concentration of <200 pg/dl, borderline deficiency as 200 - 300 pg/dl and >300 pg/dl as normal. Differences in vitamin B12 levels between different groups were assessed using Mann Whitney U test and P<0.05 was considered as statistically significant. RESULTS Vitamin B12 deficiency and borderline deficiency were recorded in 8.6% and 26.0% of the patients respectively. Vitamin B12 level was significantly lower in patients who have been on metformin for ≥10 years compared with patients with <10 years history of metformin use. Similarly, patients who were on metformin at a dose of >1000 mg/day had significantly lower vitamin B12 level when compared with patients on ≤1000 mg/day. CONCLUSION Low serum vitamin B12 level is associated with longer duration and higher dose of metformin use. Therefore, routine determination of vitamin B12 level in patients with T2DM on high dose of metformin and those with prolonged use of metformin might help in identifying patients that would benefit from vitamin B12 supplements.