Mechanical
Common causes of oral ulceration include rubbing on sharp edges of teeth, fillings, crowns, false teeth (dentures), or braces (orthodontic appliances), or accidental biting caused by a lack of awareness of painful stimuli in the mouth (e.g., following local anesthetic used during dental treatment, which the person becomes aware of as the anesthetic wears off).Eating hard foods (e.g., potato chips) can damage the lining of the mouth. Some people cause damage inside their mouths themselves, either through an absentminded habit or as a type of deliberate self-harm (factitious ulceration). Examples include biting the cheek, tongue, or lips, or rubbing a fingernail, pen, or toothpick inside the mouth. Tearing (and subsequent ulceration) of the upper labial frenum may be a sign of child abuse (non-accidental injury).Iatrogenic ulceration can also occur during dental treatment, where incidental abrasions to the soft tissues of the mouth are common. Some dentists apply a protective layer of petroleum jelly to the lips before carrying out dental work to minimize this.The lingual frenum is also vulnerable to ulceration by repeated friction during oral sexual activity ("cunnilingus tongue"). Rarely, infants can ulcerate the tongue or lower lip with the teeth, termed Riga-Fede disease. Thermal and electrical burn
Thermal burns usually result from placing hot food or beverages in the mouth. This may occur in those who eat or drink before a local anesthetic has worn off. The normal painful sensation is absent and a burn may occur.
ulceration of the genital mucous membranes, eye lesions or digestive problems, swollen glands in neck (lymphadenopathy) or a  general unwell feeling.The diagnosis comes mostly from the history and examination, but the following special investigations may be involved: blood tests (vitamin deficiency, anemia, leukemia, Epstein-Barr virus, HIV infection, diabetes) microbiological swabs (infection), or urinalysis (diabetes). A biopsy (minor procedure to cut out a small sample of the ulcer to look at under a microscope) with or without immunofluorescence may be required, to rule out cancer, but also if a systemic disease is suspected. Ulcers caused by local trauma are painful to touch and sore. They usually have an irregular border with erythematous margins and the base is yellow. As healing progresses, a keratotic (thickened, white mucosa) halo may occur. : 52

Differential diagnosis
Due to various factors (saliva, relative thinness of oromucosa, trauma from teeth, chewing, etc. ), vesicles and bullae which form on the mucous membranes of the oral cavity tend to be fragile and quickly break down to leave ulcers. Aphthous stomatitis and local trauma are very common causes of oral ulceration; the many other possible causes are all rare in comparison. Traumatic ulceration
Most mouth ulcers that are not associated with recurrent aphthous stomatitis are caused by local trauma. The mucous membrane lining of the mouth is thinner than the skin, and easily damaged by mechanical, thermal (heat/cold), chemical, or electrical means, or by irradiation.