Oral ulcers, also known as "mouth sores", are superficial ulcers that occur on the oral mucosa. They can range in size from rice grains to soybeans, are round or oval, and have a concave ulcer surface with surrounding congestion. Ulcers are cyclical, recurrent, and self-limiting, and are more likely to occur on the lips, cheeks, and tongue edges. The cause and pathogenic mechanism are still unclear. The triggers may be local trauma, mental stress, food, drugs, changes in hormone levels, and vitamin or trace element deficiencies. Systemic diseases, genetics, immunity, and microorganisms may play an important role in their occurrence and development. Treatment is mainly local, and severe cases require systemic treatment.