Medically reviewed by Dr. Sam Wood
A canker sore (aphthous ulcer) is a small, shallow, painful ulcer that forms on the soft tissue inside the mouth: the inner lips and cheeks, under the tongue, on the gums, or on the soft palate. It looks like a round or oval white or yellowish spot with a red border, usually 2 to 8 millimeters across. Canker sores are not contagious, are not caused by a virus, and heal on their own in 7 to 14 days. Over-the-counter numbing gels, salt-water rinses, and avoiding trigger foods ease the pain; a dentist can prescribe stronger treatments for large, frequent, or slow-healing sores.
Canker sore vs. cold sore
People confuse the two constantly, and they are entirely different.
| Canker sore | Cold sore (fever blister) | |
|---|---|---|
| Location | Inside the mouth, on soft tissue | Outside, on or around the lips |
| Appearance | White or yellow ulcer with a red halo | Cluster of small fluid-filled blisters that crust |
| Cause | Not fully known; immune-related | Herpes simplex virus (HSV-1) |
| Contagious | No | Yes |
| Warning sign | Sometimes a tingle a day before | Tingling, burning before blisters appear |
| Treatment | Symptom relief; steroids for severe cases | Antiviral cream or tablets |
If the sore is on the outside of your lip and started as blisters, it is a cold sore. If it is inside your mouth and is a flat ulcer, it is a canker sore.
Types of canker sores
- Minor (about 80 percent of cases): under 1 centimeter, heal in 7 to 14 days without scarring.
- Major: larger than 1 centimeter, deeper, can take two to six weeks to heal and may leave a scar. More painful and more likely to need prescription treatment.
- Herpetiform: clusters of dozens of pinpoint ulcers that can merge into one larger sore. Despite the name, they are not caused by herpes. Heal in one to two weeks.
What causes canker sores
The precise cause is not known. They are thought to be an immune reaction in the lining of the mouth, and they run in families. Common triggers:
- Minor injury: biting the cheek, a sharp tooth or filling, braces, a hard toothbrush, hot food
- Stress and fatigue
- Certain foods: citrus, tomatoes, pineapple, chocolate, coffee, nuts, spicy or very salty foods
- Sodium lauryl sulfate (SLS), the foaming agent in most toothpastes; some people get fewer sores with SLS-free toothpaste
- Hormonal changes, including the menstrual cycle
- Nutritional deficiencies: iron, vitamin B12, folate, zinc
- Quitting smoking, which can temporarily increase sores
- Some medications, including NSAIDs and beta-blockers
Frequent or severe canker sores are occasionally a sign of an underlying condition: celiac disease, inflammatory bowel disease (Crohn’s, ulcerative colitis), Behçet’s disease, or a weakened immune system. A doctor may check for these if sores are unusually persistent or accompanied by other symptoms.
How to get rid of a canker sore faster
Nothing cures a canker sore instantly; the goal is to reduce pain and help it heal on schedule.
Over-the-counter options
- Numbing gels with benzocaine (Orajel, Anbesol) or lidocaine, applied directly to the sore. Relief lasts 20 to 60 minutes.
- Protective pastes that form a film over the ulcer and shield it from food and teeth.
- Antiseptic mouthwashes with hydrogen peroxide or chlorhexidine (prescription) to keep the sore clean.
- Salt-water rinse: half a teaspoon of salt in a cup of warm water, several times a day.
- Baking soda rinse or paste: neutralizes acid and soothes.
- Milk of magnesia dabbed on the sore, alone or alternated with liquid antihistamine, a common dentist-recommended combination.
- Pain relievers: ibuprofen or acetaminophen if needed.
Prescription options (from a dentist or doctor) for large, painful, or frequent sores: topical corticosteroid gels or rinses, a chlorhexidine mouthwash, an amlexanox paste, or, for severe recurrent cases, oral medications. A dentist can also cauterize a sore chemically with silver nitrate or with a laser, which numbs the pain quickly and may shorten healing.
What to avoid while it heals: acidic, spicy, salty, and rough foods; alcohol-based mouthwash; toothpaste with SLS; touching or picking at the sore.
Canker sores in children
Children get canker sores as readily as adults, often after biting a cheek or during a cold. The same rinses and protective gels help; avoid benzocaine products in children under two, and check the label for age limits on any numbing gel. A child who refuses to eat or drink because of mouth pain, has several sores at once with fever, or has sores that keep coming back should be seen by a dentist or pediatrician, since recurrent ulcers in children are occasionally the first sign of a nutritional or gut problem.
Preventing canker sores
- Switch to an SLS-free toothpaste for a few months and see if frequency drops.
- Use a soft toothbrush and brush gently.
- Get sharp teeth, rough fillings, or ill-fitting appliances smoothed. Use orthodontic wax on braces.
- Note whether particular foods precede outbreaks and reduce them.
- Ask a doctor to check iron, B12, folate, and zinc if sores are frequent.
- Manage stress and sleep, which is easier said than done but genuinely relevant.
When to see a dentist
See a dentist if a mouth sore lasts longer than two weeks, is unusually large (over a centimeter), is extremely painful or stops you eating and drinking, keeps recurring in the same spot, or is accompanied by fever, swollen lymph nodes, or sores elsewhere on the body. A sore that is painless, firm, red or white and patchy, or that bleeds easily needs to be examined promptly, because a persistent mouth lesion that does not heal is one of the warning signs of oral cancer. See a dentist also if a sharp tooth, filling, or appliance keeps causing sores; a minute of smoothing can end a cycle of ulcers.
Frequently asked questions
How long do canker sores last?
Minor canker sores heal in 7 to 14 days on their own. Major ones can take two to six weeks. A sore lasting longer than two weeks should be checked by a dentist.
Are canker sores contagious?
No. Unlike cold sores, canker sores are not caused by a virus and cannot be spread by kissing, sharing utensils, or any other contact.
What is the fastest way to heal a canker sore?
Keep it clean with salt-water or antiseptic rinses, avoid acidic and rough foods, use a numbing or protective gel for comfort, and do not irritate it. For a large or very painful sore, a dentist can prescribe a steroid gel or cauterize it, which shortens the course.
Why do I keep getting canker sores?
Recurrent canker sores usually run in families and are triggered by stress, minor injury, certain foods, or SLS toothpaste. Frequent sores can also signal low iron, B12, folate, or zinc, or conditions like celiac or inflammatory bowel disease; a doctor can check.
Can a canker sore be a sign of something serious?
Rarely. Occasional canker sores are harmless. Sores that are unusually frequent, large, slow to heal, painless, or accompanied by other symptoms deserve evaluation to rule out nutritional deficiencies, autoimmune conditions, or, for a single persistent lesion, oral cancer.
Does salt water help canker sores?
Yes. A warm salt-water rinse several times a day keeps the sore clean, reduces bacteria, and eases discomfort. It does not shorten healing dramatically but it is safe, cheap, and mildly helpful.
Sources
- Mayo Clinic – Canker sore
- Cleveland Clinic – Canker sores
- American Dental Association (MouthHealthy) – Canker sores
- National Institute of Dental and Craniofacial Research – Oral cancer
This article is for general information only and is not a substitute for professional dental advice, diagnosis, or treatment. Always consult a licensed dentist about your specific situation.