Quick takeaways Canker sores are small but can make every meal a chore. Most heal on their own, and a few simple steps can ease the sting while they do.
Most canker sores heal on their own within one to two weeks. You usually cannot make one disappear overnight, but you can make it less painful and help it heal undisturbed: rinse with warm salt water, avoid foods that sting, brush gently around it, and ask a pharmacist about an over-the-counter gel, mouthwash or protective paste. A sore that lasts longer than about three weeks, keeps coming back or is unusually large deserves a dental or medical check.
What a canker sore is
A canker sore, also called an aphthous ulcer or simply a mouth ulcer, is a small, shallow, round or oval sore inside the mouth. It is usually white or yellowish in the middle with a red rim, and it tends to appear on the inside of the cheeks or lips, on the tongue or where the gums meet the inner cheek. Canker sores are not contagious. They are different from cold sores, which are caused by a virus, appear on or around the outside of the lips and can pass to other people.
Canker sore or cold sore?
| Feature | Canker sore | Cold sore |
|---|---|---|
| Where it appears | Inside the mouth | On or around the lips |
| What it looks like | A shallow ulcer with a pale centre and red edge | Small fluid-filled blisters that later crust |
| Cause | Not fully understood; triggers vary | A common virus |
| Contagious? | No | Yes, especially while blisters are present |
What causes canker sores
The exact cause is not fully understood, and often there is no obvious trigger. Things commonly linked with them include:
- small injuries, such as biting your cheek, a sharp tooth edge, rough brushing or a brace wire rubbing;
- stress, tiredness or feeling run down;
- hormonal changes, for example around periods;
- certain foods for some people, such as acidic fruit, spicy dishes or chocolate;
- toothpastes containing a foaming agent called sodium lauryl sulfate;
- stopping smoking, which can bring a short spell of mouth ulcers for some people;
- low levels of iron, vitamin B12 or folate, and some gut, immune or other health conditions, which a doctor can look into.
Soothing steps at home
- Rinse with warm salt water. Dissolve about half a teaspoon of salt in a glass of warm water, swish gently and spit. It may sting briefly, but many people find it soothing afterwards.
- Brush carefully. Use a soft brush and avoid scraping across the sore. Our guide to brushing your teeth properly describes a gentle technique.
- Choose soft, mild food. Skip very hot drinks and crunchy, salty, spicy or acidic food until it settles. A straw can help with drinks.
- Ask a pharmacist about treatments. Pharmacies sell protective pastes, numbing gels, antiseptic mouthwashes and anti-inflammatory sprays. Some are not suitable for children, in pregnancy or alongside certain medicines, so mention these when you ask.
- Find anything that rubs. If a broken tooth, denture or brace keeps catching the same spot, a dentist can smooth or adjust it.
Things that tend to make it worse
- Poking, picking or repeatedly touching the sore with your tongue.
- Strong alcohol-based mouthwashes, which can sting.
- Scrubbing with a worn, splayed brush; our article on replacing your toothbrush explains what to look for.
- Home remedies applied straight onto the sore, such as undiluted essential oils or crushed aspirin, which can burn delicate tissue.
Preventing repeat sores
- Keep a short diary of when sores appear to spot possible food or stress patterns.
- If you suspect your toothpaste, try one without sodium lauryl sulfate for a while and see whether anything changes.
- Have sharp teeth, rough fillings or rubbing appliances checked.
- Keep up a gentle daily cleaning routine. Ulcers can also leave breath less fresh, which our guide to getting rid of bad breath touches on.
Signs a mouth ulcer needs a professional look
Most mouth ulcers are harmless, but get one checked if:
- it has not healed after about three weeks;
- you get ulcers very often, or several at once;
- a sore is unusually large, very painful, bleeding or spreading;
- there is a red or white patch, or a lump, that does not go away, even if it is painless;
- you also have a fever, feel generally unwell or have sores elsewhere on the body;
- the pain stops you eating or drinking properly.
A dentist can examine the area properly and refer you on if needed. If reaching a practice quickly is difficult, some dental teams offer a first assessment remotely; our explainer on teledentistry describes how that works, although an ulcer that will not heal should still be seen in person.
One thing first
A guide, not a verdict
Our pages are good for understanding a topic and for writing down the questions you want to ask. They cannot tell you what is happening in your own mouth, lungs or body.
If anything here seems to clash with what your dentist, pharmacist or doctor has told you, their advice comes first.
The writers at Inside Schizophrenia
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Before you use this site. Everyday reading only. Your dentist, pharmacist or doctor knows your own situation.