Most mouth ulcers are harmless and heal themselves. This is about the small number that do not, and the three-week rule that separates the two. Most canker sores are small, painful and heal by themselves within one to two weeks, but a sore that lasts beyond three weeks, keeps returning in the same place, bleeds without cause or sits beside a white or red patch should be examined by a dentist or doctor promptly.
The rule worth remembering
Three weeks. An ulcer, patch or sore that has not healed within about three weeks warrants examination, not because it is likely to be serious, but because that is the threshold at which it stops being routine.
Painlessness is not reassurance. Some of the changes that most warrant assessment do not hurt at all. See oral cancer screening.
Common types
Most of what people encounter falls into a small number of categories.
| Type | Typical appearance | Usual course |
|---|---|---|
| Aphthous ulcer | Round, pale center, red border, painful | Heals in 7–14 days; can recur |
| Traumatic ulcer | At a spot matching a sharp edge or bite | Heals once the cause is removed |
| Denture-related sore | Where a denture rubs | Settles after adjustment |
| Cold sore | On or around the lip, blistering | Recurs; distinct from ulcers inside the mouth |
| Oral thrush | White patches that may rub off, sore | Needs assessment and treatment |
| Anything lasting over 3 weeks | Any appearance, painful or not | Examine: do not wait further |
What helps in the meantime
For ordinary ulcers, these reduce discomfort while healing happens.
- Avoid acidic, spicy, salty and very crunchy food
- Have any sharp tooth or denture edge smoothed
- Keep the mouth clean; a soft brush around the area
- Over-the-counter gels can numb temporarily
- Note the date it appeared, so you know when three weeks is up
Common questions
Mouth Ulcers and Sores: questions people ask
When should I worry about a mouth ulcer?
If it has not healed in about three weeks, if it keeps recurring in the same place, if it is unusually large, or if there is a lump or numbness with it. Painless does not mean harmless.
What causes recurring ulcers?
Several things are associated, including stress, minor trauma, some deficiencies and certain conditions. Frequently recurring ulcers are worth investigating rather than just treating.
Is a cold sore the same as a mouth ulcer?
No. Cold sores are viral, appear on or around the lip, and blister. Aphthous ulcers occur inside the mouth and are not the same condition.
Can dentures cause ulcers?
Yes, commonly, where an edge rubs. It usually settles after an adjustment: see the practice rather than enduring it, since persistent rubbing is its own risk.
Should I use mouthwash on an ulcer?
Strongly alcoholic rinses often sting and can irritate. Ask about a product suited to ulcers rather than reaching for a general mouthwash.
What is the three-week rule for mouth sores?
An ulcer, patch or sore that has not healed within about three weeks warrants examination. That is not because it is likely to be serious. It is the threshold at which it stops being routine. Painlessness is not reassurance. Some of the changes that most warrant assessment do not hurt at all.
What are the common types of mouth ulcers and sores?
An aphthous ulcer is round with a pale center and red border, painful, and heals in 7 to 14 days. A traumatic ulcer sits where a sharp edge or bite hits. A denture-related sore sits where a denture rubs and settles after adjustment. A cold sore is on or around the lip. Oral thrush shows white patches and needs assessment.
What helps an ordinary mouth ulcer heal comfortably?
Avoid acidic, spicy, salty and very crunchy food. Have any sharp tooth or denture edge smoothed. Keep the mouth clean with a soft brush around the area. Over-the-counter gels can numb temporarily. Note the date it appeared, so you know when three weeks is up.