Metformin is the first-line treatment for type 2 diabetes mellitus (T2DM) and has additional benefits such as anti-aging, antiviral, and anti-inflammatory effects. While the liver was traditionally considered the main site of metformin's glucose-lowering action, recent evidence suggests the gastrointestinal tract (GIT) also plays a significant role. Metformin affects glucose uptake and absorption in the GIT, increases glucagon-like peptide-1 (GLP-1) secretion, alters gut microbiota composition, and modulates immune responses. However, it can cause gastrointestinal side effects in up to 20% of patients, including nausea, vomiting, and diarrhea, and may lead to lactic acidosis and vitamin B12 deficiency. Metformin's mechanisms of action in the GIT include inhibiting sodium-glucose cotransporter 1 (SGLT1), increasing GLUT2 translocation, and modulating gut microbiota. It enhances GLP-1 secretion through various pathways, including inhibiting DPP4, altering bile acid levels, and activating G-protein-coupled receptors. Metformin also influences the gut microbiota by promoting short-chain fatty acid (SCFA) production, which improves host metabolism. Additionally, it modulates immune responses by altering gut microbiota composition and suppressing pro-inflammatory cytokines. Metformin is absorbed in the small intestine via transporter proteins such as OCT1, PMAT, and SERT. Its effects on the GIT are crucial for its glucose-lowering action, with the gut-liver and gut-liver-brain axes playing key roles. However, the primary site of action remains the GIT. Metformin's adverse effects include gastrointestinal discomfort, lactic acidosis, and vitamin B12 deficiency, which may be related to gut microbiota changes and transporter gene variations. In conclusion, metformin's therapeutic effects are largely mediated through the GIT, with significant impacts on glucose metabolism, GLP-1 secretion, gut microbiota, and immune responses. Understanding these mechanisms is essential for optimizing its use and managing its side effects.Metformin is the first-line treatment for type 2 diabetes mellitus (T2DM) and has additional benefits such as anti-aging, antiviral, and anti-inflammatory effects. While the liver was traditionally considered the main site of metformin's glucose-lowering action, recent evidence suggests the gastrointestinal tract (GIT) also plays a significant role. Metformin affects glucose uptake and absorption in the GIT, increases glucagon-like peptide-1 (GLP-1) secretion, alters gut microbiota composition, and modulates immune responses. However, it can cause gastrointestinal side effects in up to 20% of patients, including nausea, vomiting, and diarrhea, and may lead to lactic acidosis and vitamin B12 deficiency. Metformin's mechanisms of action in the GIT include inhibiting sodium-glucose cotransporter 1 (SGLT1), increasing GLUT2 translocation, and modulating gut microbiota. It enhances GLP-1 secretion through various pathways, including inhibiting DPP4, altering bile acid levels, and activating G-protein-coupled receptors. Metformin also influences the gut microbiota by promoting short-chain fatty acid (SCFA) production, which improves host metabolism. Additionally, it modulates immune responses by altering gut microbiota composition and suppressing pro-inflammatory cytokines. Metformin is absorbed in the small intestine via transporter proteins such as OCT1, PMAT, and SERT. Its effects on the GIT are crucial for its glucose-lowering action, with the gut-liver and gut-liver-brain axes playing key roles. However, the primary site of action remains the GIT. Metformin's adverse effects include gastrointestinal discomfort, lactic acidosis, and vitamin B12 deficiency, which may be related to gut microbiota changes and transporter gene variations. In conclusion, metformin's therapeutic effects are largely mediated through the GIT, with significant impacts on glucose metabolism, GLP-1 secretion, gut microbiota, and immune responses. Understanding these mechanisms is essential for optimizing its use and managing its side effects.