Canker sores: the small mouth ulcers with an outsized sting
Last updated September 3, 2026.
Canker sores (aphthous ulcers) are small, round, white-or-yellow ulcers with a red border that form on the soft inside of the mouth, and they punch far above their size in pain. They are not contagious (unlike cold sores, which sit on the lip border and are viral), not caused by infection, and they heal on their own in one to two weeks. About one in five people get them recurrently, and for them the game is trigger management.
What do they feel like?
A burning or tingling spot a day or two before a small round crater appears: white-grey or yellow center, angry red rim, on the inner cheeks, lips, tongue sides, or soft palate. Pain peaks in the first days and is provoked by acidic, spicy, salty, or rough food, and by talking and toothbrushing. Minor sores (under a centimeter) heal in 1-2 weeks without scarring; the rare major ones are larger, deeper, slower, and can scar.
Why do they come back?
The cause is immune-mediated, not infectious, and the triggers list is well-mapped: minor mouth injury (cheek bites, sharp teeth, braces, over-enthusiastic brushing), stress and fatigue, certain foods (citrus, tomatoes, chocolate, nuts, spicy or salty), toothpaste with sodium lauryl sulfate (a real and fixable trigger), hormonal shifts, and stopping smoking (paradoxically, quitting triggers them temporarily). Recurrent crops also flag deficiencies (iron, B12, folate) and, less often, celiac or inflammatory bowel disease, which is why very frequent sores deserve a blood test.
What actually helps?
- Protect and numb: pharmacy ulcer gels and protective pastes (which coat the sore), and antiseptic or anesthetic mouthwashes for the bad days.
- Salt-water rinses: half a teaspoon of salt in warm water, a few times daily: cheap, stinging briefly, and genuinely helpful.
- Dodge the triggers: skip acidic, spicy, salty, and sharp foods during a flare; if you get sores recurrently, trial an SLS-free toothpaste and keep a food diary.
- Steroid pastes for the frequent: a hydrocortisone pellet or prescription steroid paste, applied early at the tingle stage, shortens attacks; frequent sufferers should ask.
- Do not pick: they heal from the edges; trauma restarts the clock.
When is a mouth ulcer not just a canker sore?
This is the question that matters: any single ulcer that does not heal within three weeks needs a dentist or doctor to look at it, because persistent ulcers are how mouth cancers declare themselves, especially with smoking, heavy alcohol, or a lump, red or white patch, or numbness. Also get seen for: ulcers that are unusually large, very frequent (new ones before old ones heal), arriving with fever or genital ulcers or eye inflammation, or in someone immunosuppressed. And ulcers plus weight loss or fatigue earn blood tests. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Are canker sores the same as cold sores?
No, and the distinction is practical: cold sores are herpes-virus blisters on or around the lip border (and occasionally the nose or chin), are contagious, and crust over; canker sores are non-infectious ulcers inside the mouth (inner cheeks, inner lips, tongue sides, soft palate), never contagious, and never on the lip border. If the sore is inside the mouth and you have had them since childhood with triggers like stress and cheek bites, it is aphthous. If it is a cluster of blisters on the lip that tingles first, it is a cold sore, and the treatments differ (antivirals versus protective gels).
Why do I keep getting them?
The honest answer is susceptibility plus triggers: about one in five people are prone, often with family members who are too, and the immune system over-responds to tiny mouth injuries and certain foods. The fixable triggers are worth hunting: toothpaste with sodium lauryl sulfate (switching to SLS-free reduces attacks measurably in trials), cheek biting and sharp teeth or dental work, stress and sleep debt, and individual food triggers (citrus, tomatoes, chocolate, nuts are the common ones: a diary finds yours). If your frequency is increasing, ask for the deficiency blood tests: iron, B12, and folate deficiencies are treatable causes.
What actually makes them heal faster?
Nothing cures them overnight, but the effective stack: apply a protective ulcer paste or gel (it coats the crater against food and friction) after meals and at bedtime; rinse with warm salt water a few times daily; start a steroid pellet or prescription paste at the first tingle if you get them often (early steroid shortens attacks best); avoid acidic, spicy, salty, and sharp foods for the week; and use a soft toothbrush carefully around the area. With this, most minor sores are past the worst by day four or five and gone by day ten to fourteen.
Is my toothpaste really a suspect?
Genuinely: sodium lauryl sulfate, the foaming agent in most toothpastes, strips the mouth's protective mucin layer and is linked to more frequent and longer aphthous ulcers in recurrent sufferers, with trials showing fewer attacks after switching to SLS-free paste. It is the easiest experiment in this whole condition: switch for two to three months and count. SLS-free pastes are widely available now. If your ulcers cluster after dental products change, that is your answer; if nothing changes, you have ruled out the commonest fixable trigger for the price of a tube of toothpaste.
When should an ulcer be checked by a professional?
The three-week rule is the one that saves lives: a single ulcer that has not healed within three weeks gets examined by a dentist or doctor, full stop, because persistent ulcers are the presentation of mouth cancer, and the risk rises with smoking and heavy alcohol. Also get looked at for: ulcers arriving with a lump, a red or white patch that does not scrape off, numbness, or loose teeth nearby; very large or constantly recurring ulcers; ulcers with fever, genital ulcers, or eye inflammation (systemic conditions); and any ulcer pattern in someone immunosuppressed. Early-checked is nearly always fine; unchecked is the error.
Are canker sores ever a sign of something bigger?
Usually they are just aphthae, but very frequent or severe crops deserve the short list: iron, B12, and folate deficiencies (blood tests, treatable); celiac disease (mouth ulcers can be its mouth calling card, especially with bloating or anemia); inflammatory bowel disease; Behcet's disease (the rare one: recurrent mouth plus genital ulcers and eye inflammation); and immune-suppressing states or medications. The trigger for investigating is frequency and company: new ulcers forming before old ones heal, several at a time every month, or ulcers with gut, joint, eye, or skin symptoms elsewhere. Three a year is background; constant is a workup.