There are no reports that a simple recurrent mouth ulcer can be fatal. A mouth ulcer, often called a recurrent aphthous ulcer, is a very common mucosal ulcerative disease of the oral cavity with typical symptoms of surrounding redness and swelling with a yellow depression in the middle, accompanied by severe pain. The main effect of mouth ulcers on patients is more intense pain, and the mouth ulcer itself is self-limiting, healing naturally even without treatment in two weeks. In addition, it is recurrent and can recur after a while, making it difficult to cure completely. Clinically, mouth ulcers are not fatal, but if they are fatal, they are usually cancerous ulcers.