The cause of canker sores is not well understood. More than one cause is likely, even for individual patients. Canker sores do not appear to be caused by viruses or bacteria, although an allergy to a type of bacterium commonly found in the mouth may trigger them in some people. The sores may be an allergic reaction to certain foods. In addition, there is research suggesting that canker sores may be caused by a faulty immune system that uses the body's defenses against disease to attack and destroy the normal cells of the mouth or tongue.

British studies show that in about 20 percent of patients, canker sores are due partly to nutritional deficiencies, especially lack of vitamin B12, folic acid, and iron.

Emotional stress and local trauma or injury to the mouth, such as sharp metal braces, brushing with hard toothbrushes, and hot foods can lead to canker sores. Smoking and dentures can also contribute to the problem.

Canker sores are caused by:

* Injuries to the mouth, as seen frequently by contact sports players. Accidentally biting the inside of the cheek or lip may also result in a canker sore

* Temperature hot food or drink may cause a canker sore in the area of the mouth that was essentially burnt by the offending substance

* Spicy and/or acidic foods often produce a canker sore as a response to the irritation these spices and acids create in the mouth

* The use of chewing (smokeless) tobacco will often cause a canker sore to develop in the area of the mouth where the tobacco is held, due to the irritating chemicals found in the addictive product

* Poor-fitting, complete or partial dentures may cause canker sores in the area of the mouth where the denture may rubbing on the tissue. The development of a canker sore is often one of the first signs that indicate the need for a denture reline or adjustment by your dentist or denturist

* Orthodontic brackets, bands, and various other orthodontic attachments will often cause canker sores to develop in area of the mouth where there is constant friction on the oral tissues. This is common when orthodontic treatment first begins and may occur after each subsequent adjustments, throughout each stage of treatment

* Allergies to metals such as nickel may become evident in the mouth of a person wearing orthodontic devices necessary to move the teeth. Canker sores may begin to appear adjacent to the metal attachments. This is often referred to as contact dermatitis.

* Broken teeth are often sharp and may rub on the oral tissues to produce painful canker sores. Broken restorations may also cause a similar effect on the oral tissues

* Emotional stress has been identified as a possible trigger that may cause the development of canker sores

* Bacteria responsible for peptic ulcers known as Helicobacter pylori, has been linked to canker sore occurrence

* Vitamin deficiencies, specifically vitamin B12, foliate (folic acid), and iron, may trigger canker sore development

* Hormonal changes, notably during menstruation, pregnancy, and menopause, have been linked to canker sores

* Canker sores often run in families. Genetics my be a factor for many canker sore sufferers

* Allergic reactions and sensitivities to certain foods may cause a canker sore to develop. Allergic reaction to certain types of bacteria found in the mouth may also result in this type of mouth ulcer

* Celiac disease sufferers may experience canker sores. Gluten may be associated with the development of canker sores in those with celiac disease

* Information associated with inflammatory bowel disease (IBD) often will list canker sores as a complication associated with Crohn's disease and ulcerative colitis

* Mouth sores are a common occurrence observed in immuno-suppressed patients, such as those with HIV/AIDS

* Behcet's Disease, a rare autoimmune disease that damages the body's blood vessels notes mouth sores, more specifically canker sores, as a one of the four most common symptoms of the disease

* Sodium lauryl sulfate is a common ingredient in toothpaste and had been associated with the development of canker sores