As many as 22% of people with type 2 diabetes could have vitamin B-12 deficiency.

This BMJ study evaluated the effects of metformin on the incidence of vitamin B-12 deficiency (lower than 150 pmol/l), low concentrations of vitamin B-12 (150-220 pmol/l), and folate and homocysteine concentrations in patients with type 2 diabetes receiving treatment with insulin.

Compared with placebo, metformin treatment was associated with a decrease in vitamin B-12 concentration of -19%.

The absolute risk of vitamin B-12 deficiency (lower than 150 pmol/l) at study end was 7.2 percentage points higher in the metformin group than in the placebo group with a number needed to harm of 13.8 per 4.3 years.

Long term treatment with metformin may increase the risk of vitamin B-12 deficiency, which results in raised homocysteine concentrations. Vitamin B-12 deficiency is preventable; therefore, regular measurement of vitamin B-12 concentrations during long term metformin treatment should be considered.