Premenstrual oral ulcers may be caused by the presence of abnormal progesterone and prolactin levels during the luteal phase, causing dysfunction of the body’s immune system, which can lead to the development of oral ulcers. Abnormal immune function is one of the possible factors in the development of oral ulcers. Progesterone is an immunosuppressant, and the presence of progesterone receptors in the oral mucosal tissues is one of its target organs, so if there is an abnormality in progesterone, it can lead to an imbalance in the balance of the immune system, which can lead to damage to the oral mucosa. Prolactin is also involved in immunoregulation, and elevated levels of prolactin may cause immune system dysfunction, leading to oral ulcers. There may be other causes of canker sores before menstruation, so it is advisable to go to the hospital and receive professional treatment as soon as possible.