Metformin therapy and assessment for vitamin B12 deficiency: is it necessary?

Metformin therapy in type 2 diabetes mellitus (T2DM) has been recognised as a cause of vitamin B 12 deficiency for at least 40 years, but routine measurement is not currently advocated in clinical guidelines. Assessment might be of particular relevance in T2DM complicated by peripheral neuropathy. This service review examined whether serum vitamin B 12 levels were measured in patients with high dose (>2g/day) and long‐term (four years) metformin treatment, in particular among those with peripheral neuropathy. We also evaluated the effectiveness of vitamin B 12 replacement when levels were low. Of 283 patients on high dose metformin for more than four years only, 70 (25%) had vitamin B 12 levels checked. All of these 70 cases had peripheral neuropathy. Vitamin B 12 deficiency (<150pg/ml) was recorded in 23 (33%). Where vitamin B 12 levels were deficient, replacement vitamin B 12 was documented in only two (2.9%) patients and improvement in neuropathic symptoms post treatment were documented in only four (5.7%) patients. Conclusion: vitamin B 12 levels were measured infrequently in T2DM, in particular among those with peripheral neuropathy. Levels were frequently low when assessed among T2DM patients with peripheral neuropathy. A record that vitamin B 12 therapy was initiated was only made in a small number of cases, so the impact on peripheral neuropathy was unclear. Recommendations: all patients with T2DM on long‐term treatment with high dose metformin should be assessed for vitamin B 12 deficiency, particularly if complicated by peripheral neuropathy, and then considered for parenteral vitamin B 12 replacement if deficient. Copyright © 2011 John Wiley & Sons.