Dark spots on the mouth are mostly caused by nevi, localized pigmentation and intestinal polyp syndrome. 1. Pigmented nevus, mostly grows on the skin or mucous membrane, if grows on the mouth, it is recommended to timely excise and perform pathological biopsy to check whether there is a possibility of malignant transformation. 2. Hyperpigmentation, after the body reaches a certain age, the pigment will be deposited on the skin or mucous membrane, which generally belongs to benign changes and only affects the aesthetics. 3. Intestinal polyp syndrome, due to polyps in the small intestine, affecting the normal digestive function, will lead to lip hyperpigmentation, if there is a tendency to malignant changes need to be dealt with in a timely manner. It is recommended that patients consult a regular hospital in a timely manner, under the guidance of a professional physician to carry out relevant examinations, and comply with the doctor’s instructions for targeted treatment.