Metformin (a biguanide) is the universal first-line pharmacotherapy for type 2 diabetes mellitus per ADA/EASD guidelines, and understanding its mechanisms of action explains both its therapeutic benefits and its unique safety profile. Metformin's primary mechanism is suppression of hepatic glucose production (gluconeogenesis) through activation of AMP-activated protein kinase (AMPK) — a master cellular energy sensor. AMPK activation occurs when metformin inhibits mitochondrial complex I of the electron transport chain, modestly reducing ATP production and increasing the AMP:ATP ratio. Activated AMPK phosphorylates and downregulates key gluconeogenic transcription factors (CREB-regulated transcription coactivator 2, TORC2), reducing expression of gluconeogenic enzymes (phosphoenolpyruvate carboxykinase, glucose-6-phosphatase), thereby decreasing hepatic glucose output — the primary driver of fasting hyperglycemia in type 2 diabetes. Secondary mechanisms include enhanced insulin sensitivity in skeletal muscle (increased GLUT-4 translocation to the cell membrane), reduced intestinal glucose absorption, and favorable effects on the gut microbiome that enhance incretin (GLP-1) secretion. Metformin does not stimulate insulin secretion from pancreatic beta cells, which is why it does not cause hypoglycemia as monotherapy — a critical clinical advantage. Additional benefits beyond glycemic control include weight...