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Background: Studies have shown an association between metformin use and vitamin B12 deficiency. Since 2017, the American Diabetes Association (ADA) Standards of Medical Care in Diabetes Guideline has included a recommendation for periodic vitamin B12 measurements in metformin-treated patients, especially those with anemia or peripheral neuropathy. Objective: To determine the overall incidence and impact of the ADA Guideline on vitamin B12 monitoring in a veteran population on long-term metformin therapy. Methods: Retrospective chart review was performed for patients on metformin who started therapy prior to 2005 at the VA North Texas Health Care System (VANTXHCS). The primary outcome was the proportion of patients with at least 1 vitamin B12 level drawn during 2016 versus 2018. Metformin dose and duration, vitamin B12 supplementation, and incident neuropathy prescriptions or diagnosis were also analyzed. Results: Of 394 patients included for the primary outcome, 136 (34.5%) had at least 1 vitamin B12 level in 2016 versus 198 (50.3%) patients in 2018 (odds ratio: 1.94, P < .001). Of the 394 patients, 157 were diagnosed with neuropathy or prescribed a medication for neuropathy without a vitamin B12 level in the previous year or with a low level that was not supplemented. Conclusion: Vitamin B12 monitoring significantly increased between 2016 and 2018, aligning with the release of the 2017 ADA guidelines. However, a large proportion of patients were identified who were diagnosed with or treated for neuropathy without adequate vitamin B12 monitoring.