Lung cancer is a leading cause of death globally with high mortality and morbidity. Patients are often diagnosed at an advanced stage. Metformin has become a primary medication used in the clinical management of type 2 diabetes mellitus (T2DM) due to its relative safety, low cost, and effectiveness, mainly exerting its hypoglycemic effect by inhibiting hepatic gluconeogenesis and insulin resistance. Research data indicate that metformin extends the distant metastasis-free survival (DMFS) and progression-free survival (PFS) of diabetic patients with lung cancer, improving overall survival rates. Metformin lowers the risk of tumour development through various mechanisms, including the adenosine 5'-monophosphate-activated protein kinase/liver kinase B1/mechanistic target of rapamycin (AMPK/LKB1/mTOR) pathway, insulin-like growth factor-1 receptor pathway, apoptosis, and autophagy. However, research findings are not entirely consistent. This article reviews the research progress of metformin in terms of lung cancer treatment within the past few years, aiming to provide a more comprehensive understanding of how metformin exerts its anti-cancer impact and how it can be clinically applied, as well as provide new insights for lung cancer treatment.