Published 20 May 2010, doi:10.1136/bmj.c2198 Cite this as: BMJ 2010;340:c2198 Josep Vidal-Alaball , honorary research fellow , Christopher C Butler , professor of primary care medicine Editorials Reduced serum vitamin B-12 in patients taking metformin Until the clinical importance is clear, simple dietary advice could solve the problem As many as 22% of people with type 2 diabetes could have vitamin B-12 deficiency. 1 The cause of vitamin B-12 deficiency in these patients remains controversial, 2 but it can have important consequences and should be considered in the differential diagnosis of diabetic neuropathy. In small short term studies, treatment with metformin has been associated with reduced vitamin B-12 concentrations and an increased risk of vitamin B-12 deficiency. 3 4 Kooy and colleagues conducted a trial of the long term effects of metformin treatment on metabolic and microvascular and macrovascular complications in patients with type 2 diabetes who were already ...