A broken corner of the mouth may be caused by a dry, windy, cold environment and a lack of water in the body, which may lead to a rupture of the corner of the mouth. In addition, if the corner of the mouth is broken and there are symptoms of redness, swelling, erosion and surface flaking, it is a sign of stomatitis. This is often caused by a deficiency of vitamins and trace elements in the body, and is common in patients who are picky eaters and partial eaters, where a lack of B vitamins, vitamin C, zinc and iron causes ulcers in the mucous membrane of the corners of the mouth. In addition, the corner of the mouth may also be suffering from herpes labialis, herpes labialis is caused by the infection of the herpes simplex virus when the human organism has a low immune function, the disease will first start a small blister at the corner of the mouth, and then broken, leaving a red, swollen, vile mucosal surface of the corner of the mouth. Patients with herpes labialis can apply acyclovir ointment to the broken area at the corner of the mouth for local antiviral treatment.