Mechanism
Metformin reduces B12 absorption in the terminal ileum (calcium-dependent mechanism). 10-30% of long-term metformin users develop B12 deficiency. Risk increases with dose and duration.
Recommendation
Check serum B12 and MMA annually. Supplement 1000-2000μg B12 (methylcobalamin or cyanocobalamin) daily if levels low or borderline. B12 deficiency mimics diabetic neuropathy — may be misdiagnosed.