To the Editor In their Clinical Update on metformin, Drs Flory and Lipska succinctly highlighted metformin鈥檚 robust safety data and low cost, rendering it a good first-line pharmacologic treatment for type 2 diabetes in most patients.1 We agree with their conclusion but wish to mention metformin-induced vitamin B12 deficiency, an elusive yet common and potentially reversible adverse effect of long-term metformin use that may have implications for patients鈥� quality of life.