Potential Causes of Canker Sores

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Canker sores are very painful sores found inside the mouth that often appear out of nowhere, leaving you wondering what might be causing them. Canker sores (aphthous ulcers) are not contagious and are not related to the herpes simplex virus, also known as cold sores.

The exact cause of canker sores is still unknown. They happen when your immune system reacts to attack the lining of your mouth. This may be triggered by mechanical irritation or trauma, as an allergic reaction to something you ate, or exposure to chemicals in toothpaste or mouthwash. In some cases, certain types of bacteria or viruses are responsible for the painful mouth sores.

Women statistically suffer from canker sores more often than men. Canker sores are typically seen in people between the ages of 10 and 40, although they have been known to show up at any age.

At-home and OTC canker sore remedies
Illustration by Brianna Gilmartin, Verywell


Canker sores are thought to be caused by or triggered by these factors:

Trauma to the Mouth

  • Injuries to the mouth, as seen frequently by contact sports players
  • Accidentally biting the inside of the cheek or lip
  • Hot food or drink burning the inside of the mouth​​
  • 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.
  • 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 adjustment by your dentist or denturist.
  • Orthodontic brackets, bands, and various other orthodontic attachments will often cause canker sores to develop in an 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 adjustment, throughout each stage of treatment.

Irritation From Food, Drink, Tobacco, and Chemicals

  • Spicy or acidic foods
  • 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. This may be due to the irritating chemicals found in the addictive product.
  • Sodium lauryl sulfate is a common ingredient in toothpaste and had been associated with the development of canker sores.

Click Play to Learn About the Potential Causes of Canker Sores

This video has been medically reviewed by Brian T. Luong, DMD

Allergic and Immune Reactions

  • 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.
  • 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.
  • People with celiac disease may experience canker sores. Gluten may be associated with the development of canker sores in those with celiac disease.

Miscellaneous Causes

  • Emotional stress has been identified as a possible trigger that may cause the development of canker sores.
  • Bacteria responsible for peptic ulcers has been linked to canker sore occurrence.
  • Vitamin deficiencies, specifically vitamin B12, folate (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 may be a factor.
  • 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 immunosuppressed patients, such as those with HIV/AIDS.
  • Behcet's disease


Treatment is generally not necessary for most canker sores as they tend to heal quickly on their own. If canker sores persist for longer than two weeks, see the dentist.

See your dentist immediately if canker sores:

  • Become unusually larger than normal
  • Are extremely painful, interfering with eating, drinking, and talking
  • Begin to appear more frequently than normal
  • Do not heal after 14 days
  • Are accompanied by a high fever
  • Appear to become infected

Frequently Asked Questions

  • Are canker sores caused by a virus?

    There are many possible causes for canker sores, and healthcare professionals don’t fully understand all the factors. It's possible that a virus could cause canker sores. It’s also possible that a pre-existing virus might put you at risk for developing a canker sore or might prevent the sore from healing quickly. 

  • Do you get canker sores from COVID?

    There is a link between COVID-19 infection and canker sores, However, there is limited research on the relationship, and more studies are needed to understand whether COVID causes the canker sores or not. 

  • Are canker sores a sign of autoimmune disease?

    There isn’t significant research connecting canker sores and autoimmune diseases. Some researchers have noticed higher rates of canker sores among those with thyroid autoimmune disorders, but those studies are small and not conclusive. 

6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Altenburg A, El-Haj N, Micheli C, Puttkammer M, Abdel-Naser MB, Zouboulis CC. The treatment of chronic recurrent oral aphthous ulcers. Dtsch Arztebl Int. 2014;111(40):665–673. doi:10.3238/arztebl.2014.0665

  2. Canker sores (mouth ulcers): Overview. InformedHealth.org [Internet]. Cologne, Germany: Institute for Quality and Efficiency in Health Care

  3. Brocklehurst P, Tickle M, Glenny AM, et al. Systemic interventions for recurrent aphthous stomatitis (mouth ulcers). Cochrane Database Syst Rev. 2012;(9):CD005411. doi:10.1002/14651858.CD005411.pub2

  4. Edgar NR, Saleh D, Miller RA. Recurrent aphthous stomatitis: a review. J Clin Aesthet Dermatol. 2017;10(3):26-36.

  5. Orilisi G, Mascitti M, Togni L, et al. Oral manifestations of covid-19 in hospitalized patients: a systematic review. International Journal of Environmental Research and Public Health. 2021;18(23):12511. doi:10.3390%2Fijerph182312511

  6. Ozdemir IY, Calka O, Karadag AS, Akdeniz N, Ozturk M. Thyroid autoimmunity associated with recurrent aphthous stomatitis: Thyroid disease with RAS. Journal of the European Academy of Dermatology and Venereology. 2012;26(2):226-230. doi:10.1111/j.1468-3083.2011.04040.x

Additional Reading

By Shawn Watson
Shawn Watson is an orthodontic dental assistant and writer with over 10 years of experience working in the field of dentistry.