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Long-term Metformin Use and Vitamin B12 Deficiency in the Diabetes Prevention Program Outcomes Study.
Aroda, Vanita R.; Edelstein, Sharon L.; Goldberg, Ronald B.; Knowler, William C.; Marcovina, Santica M.; Orchard, Trevor J.; Bray, George A.; Schade, David S.; Temprosa, Marinella G.; White, Neil H.; Crandall, Jill P.; the Diabetes Prevention Program Research Group
[Article] Journal of Clinical Endocrinology & Metabolism. 101(4):1754-1761, April 2016.

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Context: Vitamin B12 deficiency may occur with metformin treatment, but few studies have assessed risk with long-term use.

Objective: To assess the risk of B12 deficiency with metformin use in the Diabetes Prevention Program (DPP)/DPP Outcomes Study (DPPOS).

Design: Secondary analysis from the DPP/DPPOS. Participants were assigned to the placebo group (PLA) (n = 1082) or the metformin group (MET) (n = 1073) for 3.2 years; subjects in the metformin group received open-label metformin for an additional 9 years.

Setting: Twenty-seven study centers in the United States.

Patients: DPP eligibility criteria were: elevated fasting glucose, impaired glucose tolerance, and overweight/obesity. The analytic population comprised participants with available stored samples. B12 levels were assessed at 5 years (n = 857, n = 858) and 13 years (n = 756, n = 764) in PLA and MET, respectively.

Interventions: Metformin 850 mg twice daily vs placebo (DPP), and open-label metformin in the metformin group (DPPOS).

Main Outcome Measures: B12 deficiency, anemia, and peripheral neuropathy.

Results: Low B12 (<= 203 pg/mL) occurred more often in MET than PLA at 5 years (4.3 vs 2.3%; P = .02) but not at 13 years (7.4 vs 5.4%; P = .12). Combined low and borderline-low B12 (<= 298 pg/mL) was more common in MET at 5 years (19.1 vs 9.5%; P < .01) and 13 years (20.3 vs 15.6%; P = .02). Years of metformin use were associated with increased risk of B12 deficiency (odds ratio, B12 deficiency/year metformin use, 1.13; 95% confidence interval, 1.06-1.20). Anemia prevalence was higher in MET, but did not differ by B12 status. Neuropathy prevalence was higher in MET with low B12 levels.

Conclusions: Long-term use of metformin in DPPOS was associated with biochemical B12 deficiency and anemia. Routine testing of vitamin B12 levels in metformin-treated patients should be considered.