The causes of sores at the corners of the mouth are usually herpes simplex, which is common, but also caused by xerostomia. 1. Herpes simplex, generally due to type I herpes simplex virus infection. The main part of the mucous membrane of the mouth and lips and the junction of the skin appear blisters, blisters dry and break to form sores at the corners of the mouth and other manifestations. Generally in about 7 to 10 days can be self improvement, but the virus usually will exist latent, when tired, cold and other immune system will recur. You can apply acyclovir cream and other treatments under the guidance of a doctor. 2. Keratitis is an allergic disease, related to seasonal changes, vitamin deficiency or other allergens stimulation (fluoride toothpaste, etc.), mainly in the corners of the mouth erythema, vesicles, and even fissures and other manifestations. Pay attention to eat more vegetables and fruits, do not eat stimulating food to avoid triggering allergens. If the phenomenon of sores at the corners of the mouth, it is recommended to go to the hospital in time to confirm the diagnosis and timely treatment to restore health.