Long-Term Metformin Raises Risk for B12 Deficiency

Paul S. Mueller, MD, MPH, FACP
Authors and Disclosures
Posted: 07/23/2010; Journal Watch © 2010 Massachusetts Medical Society
Abstract and Introduction
Abstract
Routine B12 monitoring is recommended.
Introduction
Metformin — the most commonly prescribed oral drug for patients with type 2
diabetes — induces vitamin B12 malabsorption and therefore can raise risk for
vitamin B12 deficiency. In this multicenter randomized trial, Dutch investigators
assessed risk for B12 deficiency (blood level, <150 pmol/L), low vitamin B12 level
(blood level, 150–220 pmol/L), and elevated blood homocysteine levels in patients
with type 2 diabetes who were treated with metformin.
Overall, 390 patients received metformin (850 mg) or placebo three times daily for
4.3 years. Compared with placebo, metformin was associated with a significant 19%
decrease in baseline vitamin B12 concentration. The absolute risks for vitamin B12
deficiency and low vitamin B12 level were significantly higher in the metformin group
than in the placebo group: 7% and 11% higher, with numbers needed to harm of 14
and 9 per 4.3 years, respectively. Blood homocysteine levels were correspondingly
and significantly elevated. Notably, the effect of metformin on lowering B12 levels
increased with duration of therapy.