Originally published March 20, 2019. Updated September 17, 2026.
Canker sores, also called aphthous ulcers, are small, painful sores that form on the soft tissues inside the mouth. Research suggests they affect roughly one in five people, most often adolescents and young adults. They are not contagious and usually heal on their own within one to two weeks, but they can make eating, drinking, and talking uncomfortable. This guide explains what canker sores are, what causes and triggers them, how to relieve the pain, and when a mouth sore deserves a professional evaluation.
What are canker sores?
According to MedlinePlus, canker sores are small, round sores that can appear on the inside of the cheek, under the tongue, or at the back of the throat. They typically have a red edge and a gray center. Cleveland Clinic describes them as white, gray, or yellow ulcers with a red border. Clinicians classify them as minor, major, or herpetiform, depending on the number of sores, their size, how long they last, and how often they return.
How is a canker sore different from a cold sore?
The two are often confused, but they are different conditions. Canker sores form inside the mouth and are not contagious, as both the ADA and MedlinePlus point out. Cold sores are a separate problem, and unlike canker sores, they are contagious. If you are unsure which you have, your dentist can tell you.
What causes canker sores?
No one knows the exact cause. The American Dental Association (ADA) says immune system problems, bacteria, or viruses may be involved, and that trauma to the soft tissues of the mouth can cause a canker sore. A StatPearls clinical review describes recurrent canker sores as multifactorial, involving an inherited susceptibility along with contributing factors such as microbes, foods, medications, hormones, stress, nutritional deficiencies, and systemic disease. Up to 46 percent of patients report a family history.
What triggers canker sores?
Triggers vary from person to person, and keeping a simple log can help you spot yours. Commonly cited triggers include:
- Minor injuries: biting your cheek, dental work, or an irritating appliance.
- Stress, which MedlinePlus lists as a possible trigger.
- Hormonal changes, including those related to the menstrual cycle.
- Food sensitivities. MedlinePlus lists food allergies, and the ADA notes that hot, spicy, or acidic foods can irritate an existing sore.
- Vitamin and mineral deficiencies. The StatPearls review also links canker sores to anemia and to gastrointestinal conditions such as Crohn disease and celiac disease.
If you get canker sores often, a medical or dental evaluation can check for a nutritional or underlying cause.
How long do canker sores last?
The ADA says canker sores usually heal on their own after one or two weeks, and Cleveland Clinic gives a similar timeframe. Major canker sores are larger and can take longer to resolve. Because typical sores follow this timeline, one that lasts well beyond two weeks should be examined.
How can you relieve canker sore pain?
Treatment aims to ease discomfort while the sore heals. The ADA recommends:
- Over-the-counter topical anesthetics and antimicrobial mouthwashes for temporary relief.
- Avoiding hot, spicy, or acidic foods that can irritate the sore.
MedlinePlus similarly lists ointments, creams, and rinses for pain relief, along with skipping hot and spicy food. For more stubborn cases, the ADA notes that a dentist can prescribe antibiotics and use oral bandages to reduce secondary infection, and the StatPearls review describes topical medications as first-line treatment, with systemic medications used when needed.
How can you reduce the chance of canker sores coming back?
There is no guaranteed way to prevent them, but you can lower the odds by reducing known triggers:
- Protect your mouth: brush gently, use a soft toothbrush, and let us know if a rough filling or appliance keeps irritating your cheek.
- Eat a balanced diet, since nutrient deficiencies are a listed trigger.
- Notice whether stress, certain foods, or your cycle line up with outbreaks.
- Keep up with regular dental visits so problems with teeth or restorations are caught early.
When should you see a dentist about a mouth sore?
Make an appointment if a sore lasts longer than about two weeks, is unusually large or very painful, keeps returning, spreads, or comes with a fever or trouble eating and drinking. A sore that does not heal is one of the warning signs we cover in our guide to oral cancer screening, and it is always worth having checked. Our preventive dentistry team examines the soft tissues of your mouth at every checkup.
Frequently asked questions about canker sores
Are canker sores contagious?
No. Canker sores are not contagious, so you cannot catch one from someone else or pass one on by kissing or sharing utensils. This is a key difference from cold sores, which are contagious.
How long do canker sores last?
Most canker sores heal on their own in one to two weeks, according to the American Dental Association. Larger sores can take longer.
What causes canker sores?
The exact cause is not known. Canker sores appear to involve the immune system along with triggers such as minor mouth injuries, stress, hormonal changes, nutrient deficiencies, and certain foods. They tend to run in families.
What helps a canker sore heal?
Over-the-counter topical anesthetics and antimicrobial mouthwashes can relieve pain, and avoiding hot, spicy, and acidic foods helps keep the sore from being irritated. Most sores heal without treatment.
When should I see a dentist about a mouth sore?
See a dentist if a sore lasts longer than about two weeks, is unusually large or painful, keeps coming back, or comes with fever or trouble eating or drinking. A sore that does not heal should always be examined.
Have a mouth sore that will not heal?
If a sore has lasted more than two weeks, keeps coming back, or you are unsure what it is, our team can examine it and recommend the next step.
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Quick takeaways
- Canker sores are small, painful ulcers inside the mouth that are not contagious.
- The exact cause is unknown. Immune factors, minor injuries, stress, hormones, nutrient gaps, and foods can all play a role.
- Most heal in one to two weeks without treatment.
- Topical anesthetics, antimicrobial rinses, and avoiding spicy or acidic foods can ease pain.
- Get a sore checked if it lasts beyond about two weeks, is very large, or keeps returning.
Sources
- MedlinePlus, National Library of Medicine. Canker Sores.
- American Dental Association. Canker Sores. MouthHealthy.org.
- Cleveland Clinic. Canker Sore: Symptoms, Causes & Treatment.
- StatPearls Publishing. Recurrent Aphthous Stomatitis. NCBI Bookshelf, National Library of Medicine.
