No one knows what causes canker sores. The vast majority of people who develop canker sores do not have another problem as the cause.
Both hereditary and environmental causes of the disease have been suggested, but the exact cause is not clear.
A number of factors have been suggested to precipitate outbreaks in susceptible individuals; however, none has been proven to be the cause in all people.
Hormonal changes related to the menstrual cycle
Anxiety or stress
Drugs (including anti-inflammatory drugs, such as ibuprofen [Motrin], and beta-blockers, such as atenolol [Tenormin])
Food allergies or sensitivities (chocolate, tomatoes, nuts, and acidic foods such as pineapple, and preservatives such as benzoic acid and cinnamaldehyde)
Toothpastes containing sodium lauryl sulfate
Deficiencies of iron, folic acid, or vitamin B12 (although supplementation with iron or vitamins has not been shown to decrease the likelihood of ulcer resolution)
Some studies have suggested an association with Helicobacter pylori, the same bacteria that cause peptic ulcers. Recent studies have suggested that treatment of the Helicobacter pylori infection may improve symptoms or completely stop recurrent disease in some patients.
Recurrent canker sores have been associated with inflammatory bowel diseases, such as Crohn's disease and ulcerative colitis. In these cases, the development of canker sores may signal a flare-up of the bowel disease.
Celiac disease (celiac sprue, nontropical sprue, gluten intolerance, gluten-sensitive enteropathy) is a disease of the intestines caused by sensitivity to gluten, causes malabsorption and is associated with development of canker sores. Gluten refers to a group of proteins found in wheat, barley, and rye.
Behçet's disease is a condition characterized by canker sores, genital sores that resemble canker sores, and inflammation of the eye.
Infection with the AIDS virus also has been associated with canker sores.
It is a common misconception that canker sores are a form of herpes infection. This is not the case.