Metformin and vitamin B12 deficiency: A prevalence study.

OBJECTIVES: Guidelines recommend routine biochemical screening for vitamin B12 deficiency in metformin users. However, these recommendations are based on non-specific and outdated definitions of vitamin B12 deficiency. Using contemporary diagnostic criteria, we compared prevalence in metformin users with non-users. METHODS: In this cross-sectional study, we included adults with diabetes (type 1, 2, other types) attending two outpatient clinics in Denmark, March 2024 to October 2025. Exposure was current use of metformin. B12 deficiency was defined by plasma B12 ≤ 250 pmol/L and elevated serum methylmalonic acid. Crude and adjusted prevalence differences (aPD) and ratios (aPR) were estimated using binomial regression. RESULTS: Among 3,958 participants (median age 54 years, 57% male) prevalence of vitamin B12 deficiency was 2.3% in current users and 1.3% in non-users (aPD of 0.7% [95% CI: -0.32 to 1.7]; aPR of 1.56 [95% CI: 0.92 to 2.63]). In patients with type 2 diabetes (n=1,338, median age 62 years, 61% male) prevalence was 2.3% versus 2.0% (aPD of 0.38% [95% CI: -1.9 to 1.1]; aPR of 1.21 [95% CI: 0.55 to 2.66]). CONCLUSION: Vitamin B12 deficiency was uncommon with no meaningful excess risk among metformin users. Thus, routine screening may be redundant.