To the Editor In their Clinical Update on metformin, Drs Flory and Lipska succinctly highlighted metformin’s 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’ quality of life.

Symmetrical polyneuropathy is the most common form of neuropathy associated with diabetes and vitamin B₁₂ deficiency. A symmetrical lower extremity sensorimotor polyneuropathy in patients with diabetes on metformin may masquerade as diabetic neuropathy, prompting clinicians to add superfluous pharmacologic therapies such as gabapentin or pregabalin.