Minor physical injuries
Trauma to the mouth is a common cause of oral ulcers. A sharp edge of a tooth, accidental biting (this can be particularly common with sharp canine teeth, or Wisdom teeth), sharp, abrasive, or excessively salty food, poorly fitting dentures, dental braces or trauma from a toothbrush may injure the mucosal lining of the mouth resulting in an ulcer. These ulcers usually heal at a moderate speed if the source of the injury is removed (for example, if poorly fitting dentures are removed or replaced).
It is also common for these ulcers to occur after dental work, when incidental abrasions to the soft tissues of the mouth are common. A dentist can apply a protective layer of petroleum jelly before carrying out dental work in order to minimize the number of incidental injuries to the soft mucosa tissues.
Chemicals such as aspirin or alcohol that are held or that come in contact with the oral mucosa may cause tissues to become necrotic and slough off creating an ulcerated surface. Sodium lauryl sulfate (SLS), one of the main ingredients in most toothpastes, has been implicated in increased incidence of oral ulcers.
Viral, fungal and bacterial processes can lead to oral ulceration. One way to contract pathogenic oral ulcerations is to touch your chapped lips without having washed your hands first.
The most common is Herpes simplex virus which causes recurrent herpetiform ulcerations preceded by usually painful multiple vesicles which burst. Herpes Zoster (shingles), Varicella Zoster (chicken pox), Coxsackie A virus and its associated subtype presentations, are some of the other viral processes that can lead to oral ulceration. HIV creates immunodeficiencies which allow opportunistic infections or neoplasms to proliferate.
Bacterial processes leading to ulceration can be caused by Mycobacterium tuberculosis (tuberculosis) and Treponema pallidum (syphilis).
Opportunistic activity by combinations of otherwise normal bacterial flora, such as aerobic streptococci, Neisseria, Actinomyces, spirochetes, and Bacteroides species can prolong the ulcerative process.
Fungal
Coccidioides immitis (valley fever), Cryptococcus neoformans (cryptococcosis), Blastomyces dermatitidis ("North American Blastomycosis") are some of the fungal processes causing oral ulceration
Entamoeba histolytica, a parasitic protozoan is sometimes known to cause mouth ulcers through formation of cysts.
Many researchers view the causes of aphthous ulcers as a common end product of many different disease processes, each of which is mediated by the immune system.
Aphthous ulcers are thought to form when the body becomes aware of and attacks chemicals which it does not recognize.
Immunodeficiency
Repeat episodes of mouth ulcers can be indicative of an immunodeficiency, signaling low levels of immunoglobulin in the oral mucous membranes. Chemotherapy, HIV, and mononucleosis are all causes of immunodeficiency with which oral ulcers become a common manifestation.
Autoimmunity
Autoimmunity is also a cause of oral ulceration. Mucous membrane pemphigoid, an autoimmune reaction to the epithelial basement membrane, causes desquamation/ulceration of the oral mucosa.
Vitamin C deficiencies may lead to scurvy which impairs wound healing, which can contribute to ulcer formation. Similarly deficiencies in vitamin B12, zinc have been linked to oral ulceration.
A common cause of ulcers is Coeliac disease, in which case consumption of wheat, rye, or barley can result in chronic oral ulcers. If gluten sensitivity is the cause, prevention means following a gluten-free diet by avoiding most breads, pastas, baked goods, beers etc. and substituting gluten-free varieties where available. Artificial sugars, such as those found in diet cola and sugarless chewing gum, have been reported as causes of oral ulcers as well.
Flovent
Use of flovent without rinsing the mouth out afterwards may cause oral ulcers.
Cancer
Oral cancers can lead to ulceration as the center of the lesion loses blood supply and necroses. Squamous cell carcinoma is just one of medical conditions associated with mouth ulcers
The following medical conditions are associated with mouth ulcers:
Behçet's disease
Bullous pemphigoid
Celiac disease (gluten sensitivity)
Crohn's disease
Gingivostomatitis
Leukoplakia
oral lichen planus
Lupus erythematosus
Neutropenia
Oral thrush