Association of metformin use with vitamin B12 deficiency and peripheral neuropathy in type 2 diabetes mellitus: a systematic review and meta-analysis

Abstract Background Metformin is the first-line pharmacological therapy for type 2 diabetes mellitus (T2DM). However, long-term use has been associated with reduced serum vitamin B12 levels, which may contribute to peripheral neuropathy — a common and debilitating complication of diabetes. Objective To systematically evaluate the association between long-term metformin use, vitamin B12 deficiency, and peripheral neuropathy in adults with T2DM. Methods A PRISMA 2020-compliant systematic review and meta-analysis was prospectively registered in PROSPERO (CRD420261363992). PubMed, Google Scholar, Europe PMC, and Cochrane CENTRAL were searched from January 2009 to April 2026. Two reviewers independently screened records (Cohen’s κ = 0.82). Quality was assessed using the Newcastle-Ottawa Scale and GRADE framework. Results: Twenty studies ( n  ≈ 292,000; 15 countries) were included. Three pre-specified meta-analyses using random-effects models (DerSimonian–Laird) were conducted. Metformin use was significantly associated with increased peripheral neuropathy risk in cross-sectional pooling (k = 6; pooled OR = 1.84, 95% CI: 1.18–2.88; p  = 0.007; I²=68.9%), corroborated directionally by two cohorts (Serra aOR = 1.57; Yang aHR = 1.84). B12 deficiency significantly predicted neuropathy risk in T2DM-specific pooling (k = 4; OR = 2.09, 95% CI: 1.03–4.22; p  = 0.041), with a synergistic HR of 5.76 in metformin users with concurrent B12 deficiency (Jin et al.). Pooled B12 deficiency prevalence was 32.1% (95% CI: 29.1–35.3%; I²=0.0%) at consistent diagnostic thresholds. Conclusions: Metformin use was significantly associated with increased peripheral neuropathy risk. Vitamin B12 deficiency significantly predicted neuropathy risk in T2DM-specific pooling, and affected approximately one-third of metformin-treated patients at uniform diagnostic thresholds. A marked synergistic risk was observed when metformin use was compounded by concurrent B12 deficiency. These findings underscore the clinical urgency of routine B12 monitoring and supplementation in long-term metformin users. Trial registration: PROSPERO, CRD420261363992. Registered prospectively. Conclusions Metformin use was significantly associated with increased peripheral neuropathy risk. Vitamin B12 deficiency significantly predicted neuropathy risk in T2DM-specific pooling, and affected approximately one-third of metformin-treated patients at uniform diagnostic thresholds. A marked synergistic risk was observed when metformin use was compounded by concurrent B12 deficiency. These findings underscore the clinical urgency of routine B12 monitoring and supplementation in long-term metformin users. Trial registration: PROSPERO, CRD420261363992. Registered prospectively. Clinical trial number Not applicable. Systematic review registration This systematic review was prospectively registered in PROSPERO (International Prospective Register of Systematic Reviews) under registration number CRD420261363992, prior to commencement of the literature search. The full protocol is available at: https://www.crd.york.ac.uk/PROSPERO/view/CRD420261363992 .