A Wisconsin doctor reports a case of a 63-year-old man who developed a severe vitamin-B12 deficiency after having taken metformin (Glucophage) for five years. Replacing the metformin with sulfonylurea and taking 1000 mg of cyanocobalamin (vitamin B12) for two months reversed the deficiency. Dr. Mary Ann Gilligan estimates that 10 to 30 per cent of patients on metformin develop a vitamin B12 deficiency and points out there is some evidence that calcium supplementation will prevent it.
Archives of Internal Medicine, Vol. 162, February 25, 2002, pp. 484-85