What is a canker sore and why does it appear?

A canker sore (medically, recurrent aphthous stomatitis) is a painful round or oval ulcer inside the mouth, covered with a yellowish-white membrane and surrounded by a red halo. The first episode usually occurs in adolescence, and it is somewhat more common in women. Its exact cause is still unknown; however, there is strong evidence that an exaggerated immune response against the oral mucosa plays a role. Canker sores are not an infection and are not contagious.

The main factors reported to trigger episodes are:

• Minor trauma from cheek biting, hard brushing or dental treatment
• Periods of stress and intense fatigue
• Menstruation (episodes may decrease during pregnancy)
• Having recently quit smoking
• In some people, acidic foods such as citrus fruits

What types of canker sores are there?

It is common to feel burning or a prickling sensation in the area 1–2 days before a minor canker sore appears. Despite their name, herpetiform canker sores are not related to the herpes virus; they only look similar.

How can you tell canker sores, traumatic ulcers and cold sores apart?

Not every sore in the mouth is a canker sore. The differences between three common conditions can be summarised as follows:

• Traumatic ulcer: Develops after biting the cheek or tongue, a broken tooth, a sharp filling edge, a rubbing denture, hard or hot food, or overly hard brushing. The cause can usually be guessed, and it heals within 1–2 weeks once the cause is removed. If the irritation continues, it can persist for weeks or even months and its edges can harden.
• Canker sore: Can appear without obvious trauma, tends to recur and is mostly seen on movable, non-keratinised mucosa (inner lip, cheek, under the tongue, floor of the mouth).
• Herpes (cold sore): Caused by the herpes simplex virus and is contagious. The recurrent form usually starts on the edge of the lip, and inside the mouth mostly on fixed mucosa such as the hard palate and gums, as clusters of small blisters that then burst and become sores. The first infection can come with fever and widespread gum inflammation, especially in children.

How can you relieve a canker sore at home?

Minor canker sores heal on their own; the aim is to reduce pain during this time and avoid irritating the sore.

• Avoid spicy, hot, acidic, very hot and hard or crunchy foods for a few days.
• Use a soft-bristled toothbrush and clean the sore area gently.
• In some people, toothpastes without a foaming agent (sodium lauryl sulphate) may reduce how often episodes occur.
• Drink enough fluids and prevent your mouth from drying out.
• Use medicated gels, creams or mouthwashes on the advice of your clinician or pharmacist.

What not to do: Putting an aspirin tablet on the sore, pressing salt or baking soda onto it, or burning it with alcohol-based solutions can cause a chemical burn in the tissue and delay healing. Constantly probing or picking at the sore with your tongue also prolongs healing.

When should a mouth sore be shown to a clinician?

See a dentist if any of the following apply:

• A sore that lasts longer than two weeks and shows no sign of healing
• A sore with raised edges, that feels hard to the touch or has an irregular base
• A sore that is growing even though it is painless
• Sores larger than 1 centimetre, deep, or that heal with scarring
• A sore that bleeds easily or is accompanied by a white or red patch
• Numbness at the site of the sore or in the lip
• A lump in the neck that does not go away
• Pain severe enough to stop you eating or drinking

For sores that have not healed within 10–14 days after the source of irritation has been removed, a biopsy is usually needed to rule out a more serious condition. A biopsy is a short procedure performed under local anaesthesia; only a pathological examination can give a definitive diagnosis. Some sores that have been irritated for a long time can heal quickly after a biopsy.

What can frequently recurring canker sores be a sign of?

Most canker sores are not linked to any other disease. However, if your canker sores recur almost without a break, keep getting bigger or come with other symptoms, an underlying cause should be investigated:

• Iron, vitamin B12 or folic acid deficiency
• Coeliac disease and inflammatory bowel disease (with diarrhoea, weight loss, abdominal pain)
• Behçet’s disease: More common in Turkey than in many other countries. If recurrent mouth ulcers are accompanied by genital sores, eye redness or blurred vision, acne-like skin rashes or joint pain, be sure to see a doctor.
• Conditions and certain medicines that weaken immunity

In these cases, your dentist assesses the oral findings and, if necessary, recommends blood tests and referral to specialties such as a family doctor, internal medicine, dermatology or ophthalmology.

Frequently asked questions

Related pages

Medically reviewed by: Dr. Dt. Mehmet Fuat BozabaSpecialist in periodontology · September 2026

Has a sore in your mouth not healed for two weeks? For an examination at our clinic in Bahçelievler, Istanbul, message us on WhatsApp or call +90 544 289 66 33.

Sources: Ibsen OAC, Phelan JA. Oral Pathology for the Dental Hygienist, 6th ed., Saunders/Elsevier, 2014. Abubaker AO, Benson KJ (eds). Oral and Maxillofacial Surgery Secrets, 2nd ed., Mosby/Elsevier, 2007.

This article is for information only; diagnosis and treatment planning are determined solely by a doctor after an examination.