Diagnostic
Diagnosis of mouth ulcers usually consists of a medical history followed by an oral examination as well as examination of any other involved area. The following details may be pertinent: The duration that the lesion has been present, the location, the number of ulcers, the size, the color and whether it is hard to touch, bleeds or has a rolled edge. As a general rule, a mouth ulcer that does not heal within 2 or 3 weeks should be examined by a health care professional who is able to rule out oral cancer (e.g. a dentist, oral physician, oral surgeon, or maxillofacial surgeon). If there have been previous ulcers that have healed, then this again makes cancer unlikely. An ulcer that keeps forming on the same site and then healing may be caused by a nearby sharp surface, and ulcers that heal and then recur at different sites are likely to be RAS. Malignant ulcers are likely to be single in number, and conversely, multiple ulcers are very unlikely to be oral cancer. The size of the ulcers may be helpful in distinguishing the types of RAS, as can the location (minor RAS mainly occurs on non-keratinizing mucosa, major RAS occurs anywhere in the mouth or oropharynx). Induration, contact bleeding and rolled margins are features of a malignant ulcer. There may be nearby causative factor, e.g. a broken tooth with a sharp edge that is traumatizing the tissues. Otherwise, the person may be asked about problems elsewhere, e.g.
An ulcer is a deeper breach of epithelium compared to an erosion or excoriation, and involves damage to both epithelium and lamina propria.An erosion is a superficial breach of the epithelium, with little damage to the underlying lamina propria. A mucosal erosion is an erosion which specifically occurs on a mucous membrane. Only the superficial epithelial cells of the epidermis or of the mucosa are lost, and the lesion can reach the depth of the basement membrane. Erosions heal without scar formation.Excoriation is a term sometimes used to describe a breach of the epithelium which is deeper than an erosion but shallower than an ulcer. This type of lesion is tangential to the rete pegs and shows punctiform (small pinhead spots) bleeding, caused by exposed capillary loops. Causes
Ulcers and erosions can be the result of a spectrum of conditions including:

Those causing auto-immune epithelial damage, damage because of an immune defect (e.g., HIV, leukemia)
Infections (e.g. herpes viruses)
Nutritional disorders (e.g., vitamin deficiencies). Chemicals such as SLS (a common ingredient in many toothpastes) have been linked to mouth ulcers, and have been recognized by public health agencies such as the NHS as a risk factor. Injuries (e.g. biting of the lip, tongue, and cheek, hard foods can scrape the oral tissues, hot foods can cause burns)
Stress: When the body is stressed, the immune symptom may trigger mouth sores

Pathophysiology
The exact pathogenesis is dependent upon the cause.