What are canker sores (mouth sores)?
Canker sores, also called aphthous ulcers, are non-infectious mouth sores that appear on the soft tissues of the mouth and cause real pain when eating or speaking. Clinically, they are shallow ulcers with a white-yellow base and a red, inflamed border. They are very common, and women are affected more often than men.
Unlike cold sores (herpes), canker sores are not caused by a virus and do not form on the firm tissues attached to bone, such as the hard palate or the attached gums. They appear to result from a local immune reaction directed against the lining of the mouth; the exact trigger is still unknown, which is why there is no single treatment for everyone.
Where do mouth sores appear?
Canker sores usually appear on the soft, movable lining of the mouth: the inner cheeks, the edges of the tongue, the floor of the mouth and the soft palate. A sore on the side of the tongue usually hurts more when eating and speaking than one on the inner cheek.
They tend to form where the lining is rubbed or injured repeatedly: next to a sharp or broken tooth, a rough filling or braces. Smoothing the sharp edge or adjusting the appliance can help the sore heal and stop it from coming back in the same place. Cold sores (herpes simplex), by contrast, usually appear on the lips, the attached gums and the hard palate, a difference that helps the dentist tell them apart.
What causes canker sores?
Canker sores have several causes that can act together. Heredity plays a role: recurrent sores often run in families, probably because of genes that regulate the local immune response.
Minor injuries
Biting the cheek, brushing too hard, or sharp, hard foods such as chips or nuts can injure the lining and trigger a sore. Poorly fitting dentures or orthodontic appliances have the same effect through repeated friction.
Nutritional deficiencies and medical conditions
Low iron, vitamin B12, folic acid or zinc can make the lining more fragile. People with frequent sores should ask their physician about blood tests. Inflammatory bowel diseases such as Crohn's disease or ulcerative colitis are also associated with mouth sores.
Stress and hormones
Emotional stress, anxiety and periods of heavy workload are known triggers. Hormonal changes, during the menstrual cycle or pregnancy, can also set off sores.
Symptoms of canker sores on the tongue and in the mouth
The main symptom is a sharp, burning pain that gets worse with food, drinks or speaking, and it can be out of proportion to the size of the sore. Many people feel a burning or tingling sensation before the sore becomes visible.
The sore is usually flat, with a white-yellow base and sharp, well-defined edges, surrounded by a red border. The tissue around it may be slightly swollen and tender.
With major sores, larger than one centimeter, mild general symptoms such as fatigue or swollen neck glands can occur. In that case the dentist or doctor may consider further tests to rule out a general condition.
Types of canker sores
There are three main types of aphthous ulcers, each with different features and outlook.
Minor canker sores
The most common type, about 80% of cases. Smaller than one centimeter, they usually heal on their own within one to two weeks without a scar.
Major canker sores
Less common (about 10% of cases) but more severe: larger than one centimeter, deeper, lasting up to about six weeks and sometimes leaving a scar. They need medical follow-up, and a biopsy may be needed to rule out other conditions if they do not improve.
Herpetiform canker sores
Despite the name, they are not caused by herpes. Many tiny ulcers (up to about 100) appear in clusters and may merge into larger sores. They are rare but very painful.
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Are canker sores contagious?
Canker sores are not contagious: they are not passed on by direct contact, kissing, sharing utensils or saliva.
Cold sores caused by the herpes simplex virus (HSV), on the other hand, are highly contagious from an active sore. They start as small blisters that burst and crust, usually on the lips, gums or hard palate, and tend to come back in the same place. In babies and toddlers, mouth sores with fever are often herpes rather than canker sores and need to be seen by a pediatrician.
| Criterion | Canker sore | Cold sore (oral herpes) | Sore from an injury |
|---|---|---|---|
| Appearance | Flat ulcer, white-yellow base, red halo | Cluster of small blisters that burst and crust | Ulcer where the lining was bitten or rubbed (sharp tooth, denture, braces) |
| Location | Soft lining: inside the cheeks, edges of the tongue, floor of the mouth | Lips, gums, hard palate | Where the injury happened |
| Contagious? | No | Yes, especially with an active sore | No |
| Duration | One to two weeks (major sores: up to about six weeks) | Usually clears on its own in one to two weeks (MedlinePlus) | Usually heals once the cause is removed |
When does a mouth sore need to be checked?
See a dentist or a doctor for a mouth sore, or any of the signs below, that lasts more than two weeks (NIDCR).
Signs that need an examination
- A sore that does not heal, even if it does not hurt.
- A lump, thickening or hard, rough area on the lining of the mouth, the lips or the tongue.
- A white or red patch that does not go away.
- Bleeding, pain or numbness in the mouth with no clear cause.
- Difficulty chewing, swallowing, speaking or moving the tongue or jaw.
The examination is there to rule out a precancerous lesion or oral cancer. A warning sign does not necessarily mean cancer, and the dentist may schedule a follow-up check a week or two later before deciding on further tests (ADA MouthHealthy).
How are canker sores treated?
Canker sores usually heal on their own within one to two weeks; treatment aims mainly to relieve pain and help healing.
01
Protective gels and pastes
Gels that coat the sore and over-the-counter numbing products can give temporary relief in adults; ask your pharmacist which product suits you and how to use it. For large, recurrent or long-lasting sores, the dentist or physician may prescribe an anti-inflammatory topical product, by prescription only. Numbing gels and liquids must not be used in babies and young children unless a doctor prescribes them: the FDA warns of serious side effects at this age.
02
Frequent or severe sores
Frequent or severe sores call for a medical work-up to look for an underlying cause (deficiency, inflammatory bowel disease, general condition). The physician may then consider a general anti-inflammatory treatment, by prescription and with medical follow-up.
03
Laser therapy
Laser treatment is one of the options described in the dental literature to relieve canker sore pain, as an alternative or addition to topical care. The clinic does not use lasers: at the examination, topical care is adapted to the type and location of the sore.
Home relief for mouth sores
A gentle warm salt-water rinse is one of the simplest ways to ease a canker sore: it helps reduce swelling, cleans the area and gives temporary relief. Rinse gently, without rubbing the sore.
- Baking soda rinse: baking soda diluted in water neutralizes acidity and soothes the burning.
- Honey: some people apply it for its soothing effect. Never give honey to babies under 12 months, not even on a pacifier, because of the risk of botulism.
- Cooled tea rinses: chamomile or black tea, once cooled, can bring some relief.
- Food: avoid spicy, acidic, salty or very hot foods while the sore heals.
When recurrent sores are linked to a deficiency (iron, vitamin B12, folic acid or zinc), correcting it may reduce flare-ups; the deficiency is diagnosed with blood tests, and the supplement is chosen by your physician. Home remedies complement medical care and do not replace it for severe or persistent sores.
How to prevent canker sores from coming back
Preventing recurrent canker sores means identifying your own triggers. A simple diary of flare-ups, foods, stress and health changes helps spot patterns: some people find that tomatoes, nuts, chocolate, spicy foods or citrus fruits set off sores.
Brush gently twice a day with a soft brush; a toothpaste without sodium lauryl sulfate (SLS), a foaming agent that can irritate the mouth in sensitive people, may help. Fixing the dental causes of repeated injury also matters: smoothing sharp or chipped teeth, adjusting dentures or braces and repairing rough fillings or crowns. Managing stress, sleeping enough and eating a balanced diet can also reduce flare-ups.
See a dentist or doctor for a mouth sore that has not healed within two weeks, sores that come back more than two or three times a year, or sores with fever, diarrhea, headache or a rash.
Dr Haim Aseriel, a US-trained dentist (DDS, NYU College of Dentistry), examines mouth sores, looks for local causes such as a sharp tooth or a rough restoration, and explains the findings in English. The clinic is at 12 HaTnufa Street in Talpiot, south Jerusalem, open Sun 8:00–19:00 · Mon 9:00–19:00 · Tue–Wed 9:00–21:00 · Thu 8:30–21:00 · Fri 8:00–14:00.
Canker sores and mouth sores: frequently asked questions
Canker sores usually appear on the soft, movable lining of the mouth, such as the inside of the cheeks, the edges of the tongue and the floor of the mouth, and they are not contagious. Cold sores are caused by the herpes simplex virus (HSV), tend to appear on tissue attached to bone, such as the gums and hard palate, or on the lips, and often start as a cluster of small blisters; they are contagious.
In some people, sodium lauryl sulfate (SLS), a foaming agent found in many toothpastes, irritates the lining of the mouth. If you get recurring canker sores, you can try an SLS-free toothpaste and brush gently with a soft brush, and talk to your dentist if the sores keep coming back.
Minor canker sores, the most common type (about 80% of cases), usually heal on their own within one to two weeks without a scar. Major canker sores, more than a centimeter across, are deeper and can last up to about six weeks. A mouth sore that has not healed within two weeks should be examined by a dentist or doctor.
There is no home remedy that cures a canker sore, but gentle warm salt-water rinses, avoiding spicy, acidic or hot foods and using a soft toothbrush can ease the pain. Home care does not replace an examination when sores are large, keep coming back or last more than two weeks.
A typical canker sore hurts, has a white-yellow base with a red halo and heals on its own within one to two weeks. A suspicious sore is one that has not healed after two weeks, even if it does not hurt, or one that comes with a lump, a thickening, a white or red patch or bleeding. In that case, see a dentist or a doctor to rule out a precancerous lesion or oral cancer.
Sources
- American Dental Association (MouthHealthy), Canker Sores – View the page
- Manfredini M et al., Dermatology Practical & Conceptual, 2021 – View the study
- MedlinePlus (U.S. National Library of Medicine), Canker sore, 2025 – View the page
- U.S. Food and Drug Administration, Safely Soothing Teething Pain in Infants and Children, 2024 – View the page
- Centers for Disease Control and Prevention, Foods and Drinks to Avoid or Limit (Infant and Toddler Nutrition) – View the page
- National Institute of Dental and Craniofacial Research (NIH), Oral Cancer, 2024 – View source
- MedlinePlus (U.S. National Library of Medicine), Herpes - oral – View source
- American Dental Association (MouthHealthy), Oral Cancer – View source
