What are canker sores?

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Canker sores are small lesions that develop in the mouth tissue or gums.

The last time you had a canker sore, you likely didn’t think much about it. Maybe during meals, you avoided chewing on that side of your mouth when possible. Or, you knew to brush that part of your mouth a little gentler than usual. In most cases, canker sores are harmless, but identifying them and knowing which ones might be problematic can help maintain your oral health.

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What is a canker sore?

Even if you don’t know the true definition of a canker sore, you have likely had one at some point in your life. More than half the population experiences canker sores once or regularly in the United States.¹ Canker sores, also known as aphthous ulcers, are characterized as small lesions that develop in the mouth tissue or gums. For the most part, canker sores are harmless, and the ones you likely have experienced posed no threat to your oral health.²

For a lot of people who experience canker sores, their presence disappears within a week or two. One way to know whether or not a canker sore is problematic is to take a look at its appearance. Check with your dentist if you have unusually large or painful canker sores or canker sores that don't seem to heal.³

What do canker sores look like?

The most common minor canker sores usually have the following features:⁴

  • Oval shape with a red edge

  • Usually small and contained

  • Heals without scarring in one to two weeks

Canker sores might look different depending on the specific ulcer. Some that occur in active parts of the mouth, such as the cheek or lips, can be easy to agitate during speech or eating. If you accidentally bite a canker sore, for example, it may look increasingly irritated.

Canker sores vs. cold sores

Canker sores are sometimes mistaken for cold sores or other mouth sores, but the difference is quite simple. Cold sores can occur on the surface of your lips or mouth. Likewise, cold sores can be contagious. Neither of these things is true for canker sores.⁵

What causes canker sores?

It’s largely unknown what causes canker sores in the mouth. A few reasons why canker sores show up in your mouth may be due to:⁶

  • Bacteria

  • A form of an allergic reaction to food ingredients or components of toothpaste or mouthwash

  • Nutritional deficiencies

  • Emotional stress

Food and vitamins

A few of the causes of canker sores can be traced to food consumption and vitamins in the body. For example, some of the foods known to cause canker sores in sensitive mouths include chocolate, coffee, strawberries, eggs, nuts, cheese, and spicy or acidic foods.⁷ 


Stress from a tough time at work, interpersonal conflict, or the daily worries that come along with everyday life can manifest itself as a cause for canker sores.⁸

Bacteria and certain oral care products

Lastly, common causes for canker sores can involve bacteria and oral care products with certain troublesome ingredients. Some toothpastes and mouthwashes have ingredients which can be too harsh for sensitive mouths, the most common being sodium lauryl sulfate. The chemical isn’t just used in oral care products from time to time, but household cleaners and hair products as well.⁹ Additionally, numerous organic bacteria can enter your mouth and cause an allergic reaction.

Canker sore treatments

Treating a canker sore doesn’t have to be cumbersome. In fact, most common canker sores are fairly easy to treat with the following home, over-the-counter remedies:¹⁰

  • Topical products containing Benzocaine (the most popular being Kank-A)

  • Mouth rinse, some of which are offered over the counter

In the case of a more serious canker sore, a doctor may prescribe a more intensive treatment:

  • Oral rinses containing dexamethasone, a steroid used to keep down inflammation

  • Prescription-strength lidocaine

  • Cautery, only in severe cases, by instrument or chemical

Are canker sores dangerous to oral health?

While most canker sores will go away within a week or two, there are some instances where you might want to keep an eye on them. First, if you are getting canker sores often and regularly, then you might need to speak to a medical professional. Likewise, there are some signs that a canker sore might be escalating in terms of danger.¹¹

Signs of dangerous canker sores

The telltale signs that a canker sore is worsening to the point of damaging your oral health include:¹² 

  • Unusually large canker sores

  • Recurring sores, with new ones developing before old ones heal

  • Persistent sores, lasting two weeks or more

  • Sores that extend into the lips 

  • Pain that can’t be controlled on your own

  • Difficulty eating or drinking

  • High fever along with canker sores

When these characteristics start to form, a doctor may need to take a look. Depending on the situation and what oral health concerns you have, the cost of dental insurance may be favorable to the out of pocket costs associated with caring for a seriously infected or dangerous canker sore.

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Brought to you by The Guardian Life Insurance Company of America (Guardian), New York, NY. Material discussed is meant for general illustration and/or informational purposes only and it is not to be construed as tax, legal, investment or medical advice. It is not dental care advice and should not be substituted for regular consultation with your dentist. If you have any concerns about your dental health, please contact your dentist's office.


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  1. https://www.aaom.com/index.php%3Foption=com\_content&view=article&id=82:canker-sores&catid=22:patient-condition-information&Itemid=120 (Last accessed February 2020)

  2. https://www.mayoclinic.org/diseases-conditions/canker-sore/symptoms-causes/syc-20370615, 2018

  3. https://www.mayoclinic.org/diseases-conditions/canker-sore/symptoms-causes/syc-20370615, 2018

  4. https://www.mayoclinic.org/diseases-conditions/canker-sore/symptoms-causes/syc-20370615, 2018

  5. https://health.clevelandclinic.org/contagious-canker-sore-cold-sore/, 2016

  6. https://www.mayoclinic.org/diseases-conditions/canker-sore/symptoms-causes/syc-20370615, 2018

  7. https://www.mayoclinic.org/diseases-conditions/canker-sore/symptoms-causes/syc-20370615, 2018

  8. https://www.cedars-sinai.org/blog/canker-sores.html, 2019

  9. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4651417/, 2015

  10. https://www.mayoclinic.org/diseases-conditions/canker-sore/diagnosis-treatment/drc-20370620, 2018

  11. https://www.mayoclinic.org/diseases-conditions/canker-sore/symptoms-causes/syc-20370615, 2018

  12. https://www.mayoclinic.org/diseases-conditions/canker-sore/symptoms-causes/syc-20370615, 2018