Metformin’s Impact on Body Weight in Type 2 Diabetes: A Narrative Review of Mechanisms, Clinical Evidence, and Comparative Efficacy

Background: Metformin remains one of the most widely prescribed first-line therapies for type 2 diabetes, yet its role as a weight-modifying agent remains debated. Aim: This review evaluates the mechanistic basis, clinical evidence, comparative efficacy, and safety profile of metformin with respect to body weight outcomes in type 2 diabetes. Material and Methods: A narrative review was conducted using PubMed, encompassing publications from 1995 to 2026. Eligible study types included randomised controlled trials, observational studies, systematic reviews, and meta-analyses reporting weight-related or mechanistic outcomes relevant to metformin therapy. Results: Metformin reduces body weight through AMPK-mediated inhibition of hepatic lipogenesis, appetite suppression via incretin pathways, and gut microbiota modulation. Pooled clinical evidence demonstrates a modest but consistent weight reduction of approximately 1.5 to 2.5 kg versus placebo, with durability reported over long-term follow-up. This effect is substantially inferior to GLP-1 receptor agonists and tirzepatide, which produce reductions of 10–20% in major trials. Metformin's safety profile is characterised by manageable gastrointestinal adverse effects, negligible hypoglycaemic risk, and a clinically important association with vitamin B12 depletion on long-term use. Conclusions: Metformin should not be regarded as a primary weight loss agent but retains meaningful value as a globally accessible and well-tolerated component of metabolic management in type 2 diabetes.