Vitamin B12 Deficiency in Metformin-Treated Patients

Primary Rationale for Screening

The American Diabetes Association recommends checking vitamin B12 levels in patients with metformin use exceeding 4 years, as it is associated with vitamin B12 deficiency, which can cause or worsen peripheral neuropathy, autonomic neuropathy, and anemia 1, 2
The effect of metformin on vitamin B12 increases with time, with higher risk for deficiency noted at 4-5 years of continuous use, and annual monitoring for vitamin B12 deficiency is recommended after more than 4 years of metformin therapy 1, 2, 3

For patients on metformin >4 years with normal kidney function, the American College of Clinical Endocrinologists recommends checking vitamin B12 levels annually 1, 2

The National Kidney Foundation recommends continuing to monitor eGFR at least annually in patients with normal kidney function taking metformin, since metformin requires dose adjustment or discontinuation if eGFR declines 2, 3