Oral ulcers are mainly seen in diseases such as leukoplakia, systemic lupus erythematosus, recurrent oral ulcers, and medicated oral ulcers. However, the characteristics of oral ulcers vary from disease to disease. 1. Leukoaraiosis: oral ulcers, single or multiple, painful, also seen in genital ulcers, red eyes, erythema nodosum, pins and needles reaction, etc.; 2. Systemic lupus erythematosus: oral ulcers, mostly painless, appear when the disease is active, with multi-system damage, multiple auto-antibody formation performance; 3. Recurrent oral ulcers: painful shallow ulcers, recurrent, no scarring, no other local or systemic manifestations; 4. Medicated oral ulcers: oral ulcers appear during the course of taking medications, such as methotrexate.