Effect of Long-Term Metformin Therapy on Serum Vitamin B12, Homocysteine and Lipid Profile in Type 2 Diabetes Mellitus: A Crosssectional Study at a Tertiary Care Centre

Background: Metformin is the first-line oral agent for type 2 diabetes mellitus (T2DM), but long-term use is associated withreduced serum vitamin B12, which may in turn raise homocysteine and contribute to neuropathy and vascular risk. Data fromeastern India, where dietary B12 intake is often marginal, are limited. The objective is to compare serum vitamin B12,homocysteine and lipid profile between T2DM patients on metformin for <5 years and ≥5 years, and to examine therelationship of these parameters with duration and dose of metformin. Materials and Methods: A hospital-based crosssectionalstudy was conducted from June 2025 to March 2026 in 50 adults with T2DM receiving metformin for at least oneyear, grouped by duration of therapy (<5 years, n = 20; ≥5 years, n = 30). Fasting serum vitamin B12, homocysteine, HbA1c,fasting glucose, creatinine and lipid profile were estimated. Vitamin B12 was classified as deficient (<200 pg/mL), borderline(200–300 pg/mL) or normal (>300 pg/mL). Groups were compared using the independent t-test or Mann–Whitney U test andchi-square/Fisher exact test; associations were examined by Pearson correlation and multiple linear regression. Results: Meanage was 54.9 ± 8.5 years and 66% were male. Mean serum vitamin B12 was significantly lower in the ≥5-year group than inthe <5-year group (223.6 ± 75.5 vs 348.8 ± 47.6 pg/mL; p < 0.001), and homocysteine was significantly higher (10.8 ± 2.7 vs7.2 ± 1.9 μmol/L; p < 0.001). Vitamin B12 deficiency was present in 14/30 (46.7%) patients on long-term metformin and innone of the short-term group (p < 0.001). Duration of metformin correlated strongly with vitamin B12 (r = −0.83) andhomocysteine (r = 0.77), and B12 correlated inversely with homocysteine (r = −0.88; all p < 0.001). B12 declined withincreasing daily dose (p = 0.010). On multivariable analysis, metformin duration remained an independent predictor of vitaminB12 (β = −17.4 pg/mL per year; p = 0.022). Total cholesterol, triglycerides, HDL-C and LDL-C did not differ between groups.Conclusion: Long-term and higher-dose metformin therapy is associated with a dose- and duration-dependent reduction inserum vitamin B12 and a reciprocal rise in homocysteine, without adverse change in the lipid profile. Periodic vitamin B12monitoring is warranted in patients on metformin for five years or more.