Abstract: Metformin, as a first-line drug in the treatment of type 2 diabetes, is widely used in clinic with definite efficacy and few adverse reactions. In recent years, other effects besides hypoglycemia have been found, such as a possible therapeutic effect on mental disorders (e.g., anxiety disorder, Alzheimer's disease), cancer, and cardiovascular disease. It also has regulatory effects on intestinal microbiota. The purpose of this review is to summarize the recent investigations of metformin on mental disorders, cardiovascular diseases, cancer, and intestinal microbiota.