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 taking insulin.5 Three hundred and ninety patients were randomised to receive insulin plus metformin or insulin plus placebo. After a follow-up of 4.3 years, the study found fewer macrovascular events in the metformin group, which was largely accounted for by the beneficial effect of metformin on weight.

In the linked study (doi:10.1136/bmj.c2181) this same group of authors, but with de Jager as first author, perform a further analysis of …