An ulcer formed in the mucous membrane of the oral cavity, either individually or in groups, is known as a mouth sore. Typically, they are benign and do not have any specific cause. However, on rare occasions, an ulcer that does not heal may be a symptom of oral cancer. Mouth sores are painful, and the pain can be exacerbated by certain types of food. Pain-management medications may be prescribed during treatment, which depends on the underlying cause. In other cases, treatment may be symptomatic and aimed at preventing secondary infections until the ulcers heal on their own.