Eating sugar usually does not lead to the occurrence of mouth ulcers. The specific etiology of mouth ulcers is not clear, and may be related to immune factors, genetic factors, systemic disease factors, infectious factors, etc., as follows. 1. Immune factors: patients with mouth ulcers may have decreased cellular immune function and imbalance of T-lymphocyte subpopulation. 2. Genetic factors: through the study of genetic markers and genetic material, the development of oral ulcers has a certain genetic tendency. 3. Systemic disease factors: the development of oral ulcers may be related to gastric ulcer, duodenal ulcer, ulcerative colitis and other diseases. 4. Infectious factors: some oral ulcers can be found in the lesions of Helicobacter pylori, adenovirus, cytomegalovirus, human papillomavirus and so on. The occurrence of mouth ulcers may also be related to other factors, it is recommended to go to the hospital in a timely manner, clear causes for targeted treatment.