This article is general information for our patients, not a diagnosis. If something in your mouth hurts, changes, or worries you, have it examined.
Almost everyone gets a mouth ulcer at some point, and most are nothing to worry about. The trouble is that the harmless kind and the kind I genuinely want to see can look almost identical in a mirror — so what I weigh is less how a sore looks than how it behaves: how long it lasts, whether it keeps coming back, and whether something injured the spot or it appeared on its own.
One thing first, because it matters: none of what follows is a diagnosis on its own. A sore that points one way by its look can point somewhere else entirely once the rest is in — your history, an exam, how the area behaves over a couple of weeks, and, where it counts, a tissue sample. Reading those together, in the chair, is what settles what a sore actually is. Use this to understand yours and judge how urgent it is, not to land on an answer.
The patterns I weigh
A mouth ulcer is a small break in the soft lining where the surface has worn away, leaving a tender, often pale-centred sore; the nerves just underneath are why even a tiny one can hurt out of proportion to its size. Most fall into a handful of familiar patterns — clues that point somewhere, not verdicts.
| Sore type | Where it usually sits | Typical size | Heals in | Catching? |
|---|---|---|---|---|
| Minor canker sore | Inside the lips and cheeks, sides and floor of the tongue | Under about 10 mm | 7 to 14 days | No |
| Major canker sore | The same areas, sometimes the roof of the mouth | Over 10 mm, occasionally 2 to 3 cm | Weeks, sometimes a month or more | No |
| Cluster-type (‘herpetiform’) canker sore | Inside the lips, cheeks and tongue | Crops of 10 to 100 pinpoint sores, each 1 to 2 mm | 7 to 14 days | No |
| Traumatic ulcer | Wherever something is rubbing or was bitten | Varies with the injury | About 1 to 2 weeks once the cause is fixed | No |
| Cold sore | On the lip or skin around the mouth | Small blisters that crust over | 7 to 10 days | Yes, while weeping or crusting |
Healing time and whether it spreads to others tell me more than the look ever does.
The everyday kind: canker sores
Canker sores (aphthous ulcers) are what most people mean by “mouth ulcer” — round sores that flare up inside the mouth on their own, are not contagious, and usually heal within one to two weeks. About 1 in 4 people get them at some point. They favour the soft, movable parts — inside the lips and cheeks, the sides and floor of the tongue — and tend to have a yellow-grey centre with a red rim. Three patterns:
- Minor — by far the most common (around 8 in 10): smaller than a pencil eraser, they sting for a few days and heal in about 7 to 14 days without a mark.
- Major — larger and deeper (over 10 mm, occasionally 2 to 3 cm): they can make eating and talking genuinely uncomfortable, take weeks to heal, and may leave a small scar. Because they linger, these are worth showing me.
- Herpetiform — crops of ten to a hundred pinpoint sores, each 1 to 2 mm, that can merge into a raw patch. Despite the name they are not from the cold-sore virus; the name only describes the clustered look. They heal in about 1 to 2 weeks too.
For a minor one the plan is simple: keep it clean, ease the soreness, let it run its course. It is the sores that ignore that timetable I want to see.
Why they keep coming back
Recurring canker sores are set off by a mix of things, and they often run in families — if your parents got them, you may simply be more prone. The triggers I hear about most:
- A small injury — a cheek bite, a toothbrush jab, a sharp edge on a tooth or filling, or crunchy food catching the gum.
- Stress and tiredness — the classic is an ulcer that turns up during exams or a brutal work week.
- The foaming ingredient in many toothpastes (SLS, sodium lauryl sulfate) — it provokes them in some people; switching to an SLS-free toothpaste is a cheap experiment if you get them often.
- Certain foods — citrus, tomato, chocolate or some nuts, for some people.
- Hormonal changes — some people notice them tracking with their cycle.
One pattern worth acting on: if you get them often (a rough rule is three or more bouts a year), a simple blood test is a good idea. Up to about 1 in 5 people with frequent canker sores turn out low in iron, vitamin B12 or folate, and in a small share (roughly 3 in 100) the ulcers are the first sign of a gut condition like celiac disease. Correcting a low level can quiet the whole cycle rather than just numbing each sore — a one-off ulcer needs none of this; it is the recurring pattern that is worth investigating.
“I just bit my cheek” — is that different?
It can be. A traumatic ulcer comes from a single injury — a cheek bite, a sharp tooth, a rubbing denture — with an obvious reason sitting right where the sore is. Bite your cheek eating and it should improve over a week or so, like any small wound. The ones I want to know about have an ongoing cause — a chipped or cracked tooth with a sharp edge, a broken filling, a denture that rubs. While that edge keeps catching the same spot the ulcer cannot heal; remove the cause and it usually settles within a couple of weeks. A sore that stays after the cause is gone is one to have looked at.
Cold sore or canker sore?
Easy to mix up, but they behave differently. A cold sore comes from a virus (herpes simplex) that most people carry quietly for life, and it favours the lip or the skin around the mouth — not the lining inside. The tell is a tingle or burn at the lip edge a few hours before anything shows, then small blisters that break, weep and crust over a week to ten days. Unlike canker sores, cold sores are catching through direct contact while active. An antiviral cream from the pharmacy helps if you start it at that first tingle; once it has crusted, it does little.
One note for parents: a young child’s first run-in with this virus can look alarming — fever, very sore red gums, a mouth full of little ulcers, often with drooling and refusing food. It usually clears within ten days to two weeks, and the priority is keeping fluids up. If you are worried about a sore mouth in a young child, it is always reasonable to call us.
Easing one at home
Most ulcers heal on their own; the aim is to take the edge off while they do. For an everyday canker sore:
- Warm salt-water rinses — a teaspoon of salt in a cup of warm water, swished gently a few times a day. Cheap, and it keeps the area clean.
- Eat around it — softer, blander food for a few days, easing off the acidic or spicy things that sting.
- A pharmacy gel or rinse — protective or numbing gels, or an antiseptic rinse, can soothe it. Over-the-counter painkillers can take the edge off too; your pharmacist is the right person to match either to you, especially if you take other medicines.
- Avoid your known triggers while it heals.
If your ulcers are frequent, severe, or simply not settling, there are stronger options — a steroid gel or rinse that goes straight on the sore, or topping up a low iron or vitamin level a blood test has found. Those are calls I make after seeing the sore, not something to start blindly.
A denture rubbing a sore spot
A new or recently adjusted denture pressing on one spot is a common and very fixable cause. If you can, leave the denture out for a while to let the area recover, and book a quick adjustment so we can ease the part that is rubbing. Please do not push through it for weeks or file the denture yourself — it is easy to take off too much. Sore spots and small tweaks are a normal part of getting comfortable with a new or existing denture. But a spot that keeps rubbing even after the denture is adjusted is one to have examined.
The one rule worth remembering
Any ulcer, white patch or red patch that has not healed within two to three weeks should be examined — even if it does not hurt. This is the single most important line here, so I will say it plainly.
Most ulcers are harmless and gone within two weeks; a sore that lingers well past that, or a white or red patch that simply stays, is the mouth asking to be checked. Mouth cancers are uncommon — an estimated 8,200 Canadians a year are diagnosed with a head and neck cancer, the wider group it belongs to — and I raise it not to frighten you but because they are far more treatable when found early, and a lasting sore is one of the ways they first show. Caught at its earliest stage, around 8 or 9 in 10 people are alive and well five years on; found late, that can fall below 1 in 4. A quick look is how we confirm a stubborn sore is just a stubborn sore.
A few things shift a sore from “watch” to “have it seen”:
| What I look at | Reassuring | Worth examining |
|---|---|---|
| How long it has lasted | Healing within 2 weeks | No healing after 2-3 weeks |
| Pattern | Comes and goes, fully heals between | A single sore that just stays put |
| How it feels | Sore, then settling | Firm to the touch, or oddly painless |
| The surface | Heals back to normal lining | A white, red or mixed patch that persists |
| An obvious cause | A clear injury that is healing | No cause, or it stays after the cause is removed |
Risk is partly in your hands, too. Smoking and regular drinking are the two big risk factors, and they multiply each other rather than simply add up — someone who both smokes and drinks heavily carries roughly thirty times the risk of someone who does neither. Even so, up to about 1 in 5 mouth cancers happen in people with none of the usual risks, which is why the two-to-three-week rule is for everyone. If you have a sore like that, I will examine it and, if needed, arrange a specialist referral or a small tissue sample (a biopsy). Checking the lining of your mouth, your tongue and your gums is a routine part of every check-up we do here in Barrie.
If a sore is bad enough that you cannot eat, drink or sleep, you do not have to wait — we can have it seen promptly through our emergency dental care as soon as we can fit you in.
Most mouth ulcers are ordinary, and the right move is usually patience, salt water, and avoiding whatever sets yours off for a week or two. The part to hold onto is the safety rule: a sore that has not healed in two to three weeks deserves a look, even a painless one. If you have one that will not settle, a denture that is rubbing, or you are not sure which kind you have, book a visit or call the practice at 705-721-9229 and we will sort it out together. This article is part of our Adult Patient Information library.
