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Canker Sores: Causes and 5 Ways To Prevent Them


Dr. Andreas

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Written by

Q.F. Nayibe Cubillos Morales


Medically Reviewed by

Dr. Gustavo Assatourians D.D.S

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Canker sores are ulcer-like lesions that appear inside the mouth, specifically in soft areas such as the cheeks, tongue, the base of the gums, and the inside of the lips; they are usually quite annoying and painful, but they are not contagious and heal on their own after a week or two.

If you want to learn about mouth ulcers, you can check the article here.

This article will explain the symptoms of canker sores, what causes them, how they are diagnosed, what care should be taken, and if it is possible to prevent them.

What does a canker sore look like?

They initially present as reddish lesions; then they acquire a whitish or yellowish color in the center, surrounded by bright red edges. When healing, they look grayish in the center. 

canker sores


What are the symptoms?

One or two days before a canker sore develops, a tingling or burning sensation may occur in the area where it will appear. They are painful and in some cases can cause fever and general malaise.


Canker sores are divided into 3 types:

 Minors: They are the most frequent, representing 80% of the cases. They are observed as small lesions measuring 2 to 5mm. They heal in one to two weeks and leave no scars.

 Greater: Large and deep lesions, very painful, measuring about 1 cm. They take up to 6 weeks to heal and may leave scars.

 Herpetiformis (recurrent aphthous stomatitis): They are uncommon and are seen as very small lesions, 1–2 mm in size, that clump together to form a large ulcer. They heal in one to two weeks without scarring.


What causes canker sores?

The main cause of canker sores is unknown, but several factors can trigger them, such as:

 Trauma: Injuring yourself while brushing, using braces or devices that cause rubbing on the soft tissues of the mouth, and involuntary bites, among others.

 Sensitivity to some foods: Chocolate, peanuts, cheese, tomato, and cereals, among others.

 Alteration of the immune system: Allergies.

 Psychological alterations: Emotional stress, depression, anxiety, among others.

 Adverse drug reactions: corticosteroids, immunosuppressants, antineoplastics, non-steroidal anti-inflammatory drugs, and anti-infectives.

 Hormonal changes: During the menstrual cycle or pregnancy.

 Habits: Tobacco use.

 Nutritional deficiencies: Vitamin and mineral deficiency. For example, iron, vitamin C, zinc, folic acid, and vitamin V12, among others.

 Hereditary factors

 Diseases: For example, HIV behcet’s disease, inflammatory bowel disorders, and Celiac Disease.
Thrush can be multifactorial, that is, due to a combination of various causes.



An adequate diagnosis is necessary to determine the most effective treatment for canker sores in each case.

Initially, the dentist will take a medical history in which he or she inquires about the family and personal history, habits, systemic diseases, medical and pharmacological treatments of each patient. You will be asked questions about how long you have had the ulcers if you have had them before, and if you have similar lesions in other parts of the body, among others.

Then during a clinical examination, the dentist will evaluate the appearance of lesions, size, shape, color, and location. This is done to determine the type of lesions and make a differential diagnosis with other diseases, such as herpes virus simple, also known as a lip sore.

It is a disease caused by the type I herpes virus and is usually presented as a group of small blisters filled with fluid. They occur outside the mouth near the lips and are extremely contagious. 

canker sore



Canker sores are recurrent, that is, they can reappear, but it is possible to reduce their frequency by following these recommendations:

Good oral hygiene habits

Using a soft-bristled toothbrush, brush your teeth after each meal, floss at least once a day, and avoid mouthwashes that contain alcohol, as the soreness can worsen.

Eat healthy foods

Adopt a varied diet and eat fruits, vegetables, and foods rich in vitamin C and Omega 3.

Reduce stress

Learn techniques to reduce stress and anxiety, through sports or meditation exercises.

Protect your mouth

In case of using braces or other devices in the mouth, apply wax or ask the dentist if it is possible to reduce the friction of the device so it does not cause damage.

Pay attention to the foods you eat

Avoid consuming irritating, very salty, or hot foods and those to which you have allergies.
Chew slowly.

When to visit the dentist

Canker sores do not normally pose a risk to general health, but in some cases, it is advisable to visit the dentist:

 If a restoration is dislodged or fractured and when a prosthesis is broken or damaged so the dentist can treat the problem before a lesion appears.

 Unusually large lesions.

 Very recurrent lesions that appear more than 2 or 3 times a year.

 Canker sores that remain for more than 2 weeks.

 Canker sores that extend to the lips, that is lesions inside and outside the mouth.

 Intense pain.

 Difficulty eating, drinking, or brushing teeth.

 Fever, diarrhea, or headache.


Canker sores usually go away on their own after 2-3 weeks; Recommendations can be followed at home, such as using rinses or gels; and in some cases applying topical medications or ingesting them orally. The topic is presented in depth on our website with the different treatment options and recommendations explained.



1. Assatourians, G. (November 4, 2022). What Are Canker Sores? : Causes And 8 Useful Treatments. Channel Islands Family Dental Office.

2. Canker sores and cold sores. (2005). The Journal of the American Dental Association, 136(3), 415.

3. Canker sore. (reviewed Jun 6, 2021). MedlinePlus.

4. Canker Sore (Aphthous Ulcer). (July 11, 2005). WebMD.

5. Canker Sore (Aphthous Ulcer): What It Is, Causes & Treatment. (s. f.). Cleveland Clinic.

6. Canker sores (mouth ulcers): Overview. (August 15, 2019). Institute for Quality and Efficiency in Health Care. Updated: 2022.

7. Canker sore – Symptoms and causes. (April 3, 2018). Mayo Clinic.

8. Fever Blisters & Canker Sores. (s. f.). National Institute of Dental and Craniofacial Research.

9. Kahn, A. (December 2, 2021). Painful Sensation? Could Be a Canker Sore. Healthline.

10. MacGill, M. (June 9, 2017). Everything you need to know about canker sores. Medical news today.

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