Dark spots on one side of the mouth may be caused by hyperpigmentation, medical-origin amalgam deposits, dental caries and other conditions. 1. Hyperpigmentation: dark spots on one side of the mouth may be caused by melanin deposition. Patients who smoke for a long time will leave dark and black pigmentation in the oral mucosa. Patients who often drink coffee will also have dark spots with melanin deposits inside the mouth. 2. Medical amalgam deposits: this is a dental amalgam filling caused by oral mucosal hyperpigmentation, is a common limited melanosis, the clinical manifestations of dark gray or blue-black, flat patchy, generally adjacent to the filling. 3. Dental caries: dental caries is a chronic bacterial infection of the hard tissues of the teeth, which will cause the dental tissues to appear degeneration, decay, defects and so on. When caries cavities are formed, black pigment will be deposited in the cavities, and accompanied by symptoms of food impaction in the mouth, long-term caries without treatment will also lead to black spots in the mouth. It is recommended that patients with black spots in the mouth should seek medical treatment in time to find out the specific cause of the disease, so as not to delay the condition.