Ordinary mouth ulcers heal on their own and need little more than patience. The useful part is recognising the small number that do not follow that pattern, and knowing when that matters.
Most mouth ulcers are harmless and gone inside two weeks. The useful knowledge is not how to treat the ordinary ones — it is recognising the small number that do not follow that pattern.
The ordinary kind
Aphthous ulcers, often called canker sores, are round or oval, with a white or yellow centre and a red rim. They turn up on the inside of the cheeks and lips, under the tongue, or in the floor of the mouth — the soft, moving parts.
They hurt far more than their size suggests, and they heal in roughly 7 to 14 days without scarring. Common triggers:
- Minor trauma — biting your cheek, a toothbrush slip, a bracket or denture edge rubbing.
- Stress and poor sleep.
- Hormonal changes.
- Toothpaste containing sodium lauryl sulphate (SLS), which sets some people off repeatedly.
- Low iron, B12 or folate, which is worth checking if you get them constantly.
Cold sores are a different thing
Cold sores sit on the lip border or around the nose, start as a tingle, blister, then crust over. They are viral, they recur in the same spot, and they are contagious — which canker sores are not. If you are treating one as the other, nothing will work.
The three-week rule
This is the part worth remembering.
Any sore, ulcer, lump, or white or red patch that has not healed within three weeks should be looked at. Not because it is likely to be something serious — the great majority are not — but because the few things that are serious are far more treatable when found early, and they are painless often enough that pain is a poor guide.
The same applies to:
- A patch of numbness in the lip, tongue or cheek with no obvious cause.
- A lump or thickening you can feel that is not going away.
- A sore throat or discomfort on one side that persists.
- Difficulty swallowing, or a change in how your teeth meet.
- A tooth loosening without gum disease to explain it.
None of that is a diagnosis. It is a threshold for having someone look, which takes a few minutes.
The ones that keep coming back in the same place
If an ulcer returns to the same spot repeatedly, something physical is usually rubbing it: a sharp edge on a filling or crown, a denture that no longer fits, a bracket, or a tooth that has chipped.
A dentist can smooth or adjust that in one short visit, and it ends the cycle. People put up with these for months without realising the fix takes minutes.
What helps while it heals
- Topical gels and patches from a pharmacy, which coat the ulcer and dull it.
- Salt water rinses — warm, not hot.
- Avoid the obvious aggravators: acidic, spicy, salty, very hot, crisp-edged foods.
- Switch to an SLS-free toothpaste if you get them often. For some people this alone changes the frequency.
- Do not attack it with strong alcohol mouthwash or by scrubbing. It is an open wound.
When it is not an ulcer at all
Several other things present as sore patches in the mouth — oral thrush, lichen planus, burning mouth syndrome, and reactions to medication among them. They look different to someone who sees them regularly, and they are managed differently.
In Korea, the dental specialty for the mouth lining and facial pain is 구강내과 (oral medicine). An ordinary dental clinic handles anything caused by teeth, fillings, dentures or appliances, and refers onward when it is not.
Raising it in Korean
You do not need a sentence. 입안이 헐었어요 — ibani heoreosseoyo, "I have a sore in my mouth" — and pointing is enough to start the exam.
If it has been there a while, add 3주 넘었어요 — samju neomeosseoyo, "it has been more than three weeks." That one phrase changes how the visit is approached.
At Seoul Lab Dental: bring it up even if it seems minor — checking a sore takes a couple of minutes, and if something is rubbing and causing it, we can usually smooth that in the same visit. Explained in English.
Frequently asked questions
How long should a mouth ulcer take to heal?
An ordinary aphthous ulcer heals in roughly 7 to 14 days without scarring. Anything — a sore, ulcer, lump, or white or red patch — that has not healed within three weeks should be examined. Most such cases turn out to be nothing serious, but the few that matter are far more treatable when found early, and they are often painless.
What is the difference between a canker sore and a cold sore?
Canker sores (aphthous ulcers) appear on the soft moving parts inside the mouth — cheeks, lips, under the tongue — with a white or yellow centre and a red rim, and they are not contagious. Cold sores sit on the lip border or around the nose, start with a tingle, blister and then crust over, are viral and contagious, and tend to recur in the same spot.
Why do I keep getting ulcers in the same place?
Something physical is usually rubbing it — a sharp edge on a filling or crown, a chipped tooth, an orthodontic bracket, or a denture that no longer fits. A dentist can smooth or adjust it in one short visit, which ends the cycle. People often endure this for months without realising the fix takes minutes.
What helps a mouth ulcer heal faster?
Topical gels or patches from a pharmacy coat the ulcer and dull it; warm salt water rinses help; and avoiding acidic, spicy, salty, very hot or sharp-edged food reduces the irritation. If you get them frequently, switching to a toothpaste without sodium lauryl sulphate (SLS) changes the frequency for some people. Avoid strong alcohol mouthwash — it is an open wound.
Which clinic should I go to in Korea for a mouth sore?
A dental clinic handles anything caused by teeth, fillings, dentures or appliances, and refers onward when it is not. The dental specialty for the mouth lining and facial pain is 구강내과 (oral medicine). To raise it, 입안이 헐었어요 (ibani heoreosseoyo) — I have a sore in my mouth — and pointing is enough; add 3주 넘었어요 (samju neomeosseoyo) if it has been more than three weeks.
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