Supplements · Metabolic & Cardiometabolic
metformin causes vitamin B12 deficiency
In plain terms: Does metformin cause vitamin B12 deficiency?
Part of: 💊 metformin
Yes — long-term metformin lowers serum B12 and raises risk of biochemical deficiency in a dose- and duration-dependent way, confirmed by both the DPPOS and a placebo-controlled RCT plus meta-analyses; clinically overt deficiency is less common but real.
📅 Last reviewed: 2026-07-15 ⓘ
Evidence ladder
How far up the ladder this claim has climbed. A high consensus on a low rung means "consistent so far," not "proven in people."
Top evidence so far: All trials, pooled (Meta-analysis)
How the studies fall
The evidence (20)
| Source | Grade | Stance | Quality | Finding |
|---|---|---|---|---|
| Tahir EA et al 2026 · study_type: meta-analysis | meta-analysis | mixed | low | 5 studies; RCT B12-deficiency pooled rate 1.4% [CI -0.012,0.040]; observational pooled 18.7% [CI -0.212,0.586], I2=88%; B12 decline at 6/12mo not stat/clin significant (p>0.05) |
| Baldwin FD et al 2026 · study_type: observational | observational | supports | high | UK Biobank GWAS; B12 deficiency baseline (no metformin) 0.84-1.20% vs metformin-exposed 6.02%(GG)/7.96%(GA)/12.84%(AA) CUBN rs1801222; time-to-10%-deficient 21yr(GG) vs 11yr(AA) |
| Rangare V et al 2026 · study_type: observational | observational | supports | low | n=170 T2DM; metformin >5yr independent predictor of B12 deficiency, adjusted OR 2.85 (95% CI 1.32-6.12); concomitant pantoprazole also independent |
| Bissar L et al 2026 · study_type: observational | observational | mixed | moderate | n=246 T2DM; deficiency prevalence 7.7% (metformin) vs 4.8% (non-metformin), p=0.571 (ns); median B12 296 vs 382 pg/mL, p=0.001 (sig lower) |
| Iwańska A et al 2026 · study_type: observational | observational | supports | moderate | Children with obesity, n=500, 12% on metformin; metformin users had significantly lower B12 (398 vs 410 pg/mL, p=0.043); no non-metformin severity correlation |
| Chapman 2016 · Diabetes Metab | meta-analysis | supports | moderate | Meta-analysis: metformin associated with reduced serum B12, with dose and duration as risk modifiers. |
| Kakarlapudi Y et al] # corrected 2026-08-17 from PubMed; was [Khan 2022 · Cureus | observational | supports | moderate | Cross-sectional/cohort: higher B12 deficiency prevalence and peripheral neuropathy in metformin-treated T2D; smaller, lower-quality but concordant. |
| Out 2018 · Diabetes Obes Metab | RCT | tested-null | moderate | HOME negative control: metformin did NOT lower vitamin D (no effect), showing the B12 finding is specific rather than a generic nutrient artifact. |
| Out 2018 · J Diabetes Complications | RCT | supports | moderate | HOME post-hoc: metformin raised methylmalonic acid (functional B12 marker), confirming the deficiency is metabolically real, not just a transport artifact. |
| Niafar M et al] # corrected 2026-08-17 from PubMed; was [Liu 2015 · Intern Emerg Med | meta-analysis | supports | moderate | Meta-analysis: metformin use significantly associated with lower B12 and higher deficiency risk in T2D across studies. |
| Wakode S et al 2026 · study_type: observational | observational | supports | moderate | Prospective, n=52 newly diagnosed T2DM, metformin monotherapy 6mo: B12 506.7->485.0 pg/mL (p<0.001); homocysteine and MMA rose in parallel |
| Bhadade R et al 2026 · study_type: observational | observational | supports | moderate | n=180 (90 cases/90 cohabiting non-metformin controls); B12 206.66 vs 301.44 pg/mL (p<0.001); deficiency 40.0% vs 11.1%, OR 5.33 (95% CI 2.44-11.65) |
| Prescrire 2014 · Prescrire Int | meta-analysis | supports | moderate | Review notes metformin sometimes causes B12 deficiency leading to macrocytic anemia or peripheral neuropathy. |
| Ali HO et al 2026 · study_type: observational | observational | supports | moderate | n=257 Ugandan T2DM on metformin; deficiency 10.1%; duration >=10yr AOR 3.94 (95% CI 1.34-11.56); dose 2000mg AOR 4.32 (95% CI 1.47-12.67) |
| Aroda 2016 · J Clin Endocrinol Metab | RCT | supports | high | DPPOS: long-term metformin increased risk of B12 deficiency vs placebo (HR rising with duration), with anemia and neuropathy signals; landmark prevention-cohort confirmation. |
| Ballal A et al 2025 · study_type: observational | observational | supports | moderate | Prospective cross-sectional; statistically significant association between metformin use and B12 deficiency; mean duration to deficiency onset 13.6 years |
| de Jager 2010 · BMJ | RCT | supports | high | HOME placebo-controlled RCT (4.3y): metformin lowered B12 by ~19% and raised risk of B12 deficiency (OR ~3), establishing causality not just association. |
| Yang W et al] # corrected 2026-08-17 from PubMed; was [Out 2019 · J Diabetes | RCT | supports | moderate | HOME-trial analysis: metformin associated with lower B12, more deficiency, and links to anemia and peripheral neuropathy; reinforces clinical sequelae. |
| Rubbani 2025 · Cureus | meta-analysis | tested-null | low | Meta-analysis links long-term metformin to iron-deficiency anemia and oxidative stress, noting associated B12 depletion. |
| Tillemans MPH et al 2026 · study_type: observational | observational | supports | moderate | n=7132 hospitalized elderly; metformin use independently associated with B12 deficiency, adjusted OR 2.5 (95% CI 1.7-3.6), dose-dependent |
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