Causes

There are many processes which can lead to ulceration of the oral tissues. In some cases they are caused by an overreaction by the body's own immune system. Factors that appear to provoke them include stress, fatigue, illness, injury from accidental biting,  hormonal changes, menstruation,sudden weight loss, food allergies and deficiencies in vitamin B12, iron and folic acid. Oral ulcers are also a common result of ceased cigarette smoking, affecting about two out of five quitters.
Trauma

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 (cannine teeeth)sharp or abrasive 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 vaseline before carrying out dental work in order to minimize the number of incidental injuries to the soft mucosa tissues.

Chemical Injuries:

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.

Infection

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.
Viral

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

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 infections

Protozoans

Entamoeba histolytica, a parasitic protozoan is sometimes known to cause mouth ulcers through formation of cysts.

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.
Allergy

Contact with allergens can lead to ulcerations of the mucosa.
Dietary

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 mouth out after use may cause an oral ulcer.
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 these.
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
•  Ulcerative colitis
•  Infectious mononucleosis.

Treatment

Symptomatic treatment is the primary approach to dealing with oral ulcers.