The mechanism of vitamin B12 deficiency with metformin is undoubtedly due to malabsorption of vitamin B12 at its absorption site in the terminal ileum. Initially, it was believed that metformin caused proliferation of bacteria in the small bowel either due to an effect on intestinal motility or an increased intestinal glucose level.[8] However, the current and more likely explanation for metformin-induced vitamin B12 malabsorption and deficiency is that metformin has an effect on calcium-dependent membrane action in the terminal ileum.[9] Absorption of the vitamin B12-intrinsic factor complex is calcium dependent and metformin interferes with this absorption. In support of this hypothesis is evidence that dietary calcium supplementation reverses metformin-induced vitamin B12 malabsorption