Abstract

Metformin remains the global first-line pharmacotherapy for type 2 diabetes (T2D), yet its clinical footprint continues to evolve. Beyond hepatic gluconeogenesis suppression via AMPK-dependent and independent pathways, newer work highlights gut-centric mechanisms—microbiome remodeling, bile-acid/FXR modulation, incretin biology—and appetite regulation via GDF15. Clinically, metformin’s role in T2D is foundational, with updated guidance for chronic kidney disease (CKD) and B12 monitoring. Outside glycemia, high-quality data now support metformin to mitigate antipsychotic-induced weight gain, while new randomized evidence suggests early, short-course metformin may reduce the incidence of long COVID. In contrast, large oncology trials do not show outcome benefits in non-diabetic early breast cancer, tempering earlier optimism. In PCOS, recent international guidelines refine where metformin helps (metabolic, cycle irregularity) and where it does not (routine use in pregnancy). Evidence does not support metformin as disease-modifying therapy for MASLD/NAFLD. Ongoing aging/geroscience initiatives keep the debate alive, but remain investigational. This review synthesizes mechanistic advances, updated safety guidance, and emerging indications to inform practice in 2025.

Introduction

Since its modern adoption in the 1990s, metformin has been favored for efficacy, safety, cardiovascular neutrality, weight benefit, and low cost. Contemporary research broadens its physiological canvas while clarifying limits outside classic diabetology¹.

Mechanisms: from liver to gut–brain axis

Metformin reduces hepatic glucose production via mitochondrial complex I inhibition, AMPK activation, and downstream gene expression². Gut-centric actions include microbiome remodeling³, bile acid/FXR modulation⁴, incretin enhancement⁴, and appetite reduction mediated by GDF15⁵.

Established indications and updated guidance

Metformin remains foundational for T2D therapy⁶. ADA–KDIGO consensus recommends its use if eGFR≥30, with dose reduction if 30–44⁷. Monitor vitamin B12⁸; lactic acidosis risk is low but rises with severe CKD⁷. GI intolerance is mitigated by XR and slow titration⁹.

Beyond glycemia: strong and weak evidence

Antipsychotic-induced weight gain: meta-analyses show 3–4 kg reduction over 12–24 weeks¹⁰. Long COVID: the COVID-OUT trial showed 41% risk reduction with early metformin¹¹. PCOS: 2023 guideline supports metabolic and menstrual benefits but not routine pregnancy use¹². MASLD/NAFLD: no histologic benefit¹³. Oncology: large RCTs (e.g., MA.32) neutral¹⁴.

Special populations and practical notes

Older adults require renal and B12 monitoring⁸; XR helps tolerance⁹. Pregnancy: not first-line for GDM; selective PCOS use¹². CKD/contrast: hold and recheck eGFR per label⁷.

Controversies & frontiers

TAME trial aims to test anti-aging claims but remains investigational¹⁵. Mechanistic primacy remains debated²,³,⁵.

Practical dosing & monitoring

Initiate 500 mg daily with food; titrate to 1500–2000 mg/day. eGFR≥45 usual; 30–44 reduce; <30 avoid. Hold around contrast; check B12 periodically⁶⁷⁸.

References

1. Foretz M, et al. Update on mechanisms of action. Mol Metab. 2023.

2. Wang Y, et al. Metformin–microbiome advances. Front Pharmacol. 2024.

3. Cheng M, et al. FXR/GLP-1 gut mechanism. Signal Transduct Target Ther. 2024.

4. Zhang SY, et al. Kidney-derived GDF15 with metformin. Cell Metab. 2023.

5. ADA Standards of Care 2025—Pharmacologic Approaches to Glycemic Treatment. Diabetes Care. 2025.

6. ADA–KDIGO Consensus on Diabetes in CKD. 2022.

7. Lazarus B, et al. Metformin and risk of acidosis across eGFR strata. JAMA Intern Med. 2018.

8. FDA labeling 2024 for metformin-containing products—renal cautions/contrast guidance.

9. Meta-analyses for AIWG. Schizophrenia Bulletin. 2024–2025.

10. Bramante CT, et al. Metformin reduces long COVID. Lancet Infect Dis. 2023.

11. International PCOS Guideline 2023. J Clin Endocrinol Metab. 2023.

12. AASLD Practice Guidance for MASLD. 2023.

13. CCTG MA.32 breast cancer trial—no survival benefit. JNCI. 2023.

14. TAME (Targeting Aging with Metformin) program updates. 2024.