Mouth ulcers caused by lung cancer may be caused by the decline of human resistance. On the one hand, as a consuming disease, lung cancer itself may lead to the lowering of human resistance, on the other hand, some chemotherapy and radiotherapy drugs for lung cancer may also cause the lowering of human resistance, and the oral cavity is a relatively easy part to be infected, so lung cancer patients may often have oral ulcers. There are the following suggestions for the treatment of mouth ulcers: 1. Oral cleaning: pay more attention to oral hygiene, rinse mouth frequently and pay attention to the protection of oral mucosa. 2. Diet: avoid eating too stimulating food, and eat more fresh fruits and vegetables to supplement vitamins; ensure sufficient rest. 3. Medication: If the mouth ulcers are more serious, you can take medication under the guidance of a doctor to supplement vitamins and eliminate inflammation. It is recommended that patients use medication under the guidance of a doctor, do not self-medication.