Mouth Ulcers and Canker Sores: What Causes Them and How to Treat Them
It starts as a sting when you sip orange juice. By morning, there's a small white crater inside your cheek, and every mouthful reminds you it's there. A mouth ulcer (what Americans call a canker sore) is one of the most common things patients ask us about at Bradbury Dental Surgery. It's also one of the most misunderstood.
Most heal on their own within two weeks. Some keep coming back. A small number need a proper look. This guide covers what causes mouth ulcers and canker sores, how to settle the pain at home, and when it's time to book in.
What Is a Mouth Ulcer (and Is It the Same as a Canker Sore)?
A mouth ulcer is a small, shallow break in the soft lining of your mouth. It's usually round or oval, with a white or yellow centre and a red border. "Canker sore" is simply the American name for the same thing. The clinical term is aphthous ulcer, and when they keep returning, it's called recurrent aphthous stomatitis.
They're very common. Recurring aphthous ulcers with no known cause affect around 20 per cent of the population, according to Victoria's Better Health Channel. Better Health Channel
Canker ulcers come in three types:
Type | Size | Healing time | What to expect |
Minor | Under 1 cm (often 2–5 mm) | 7–14 days | The everyday kind, making up about 8 in 10 cases, with no scarring |
Major | Over 1 cm, deeper | Up to 6 weeks | More painful and may leave a scar. Worth getting checked |
Herpetiform | Pinhead-sized clusters of 10 to 100 | 1–2 weeks | The name is misleading because they're not caused by herpes |
So what does this mean for you? If yours is small, shallow, and sore, it's almost certainly the minor type.
What Causes Mouth Ulcers and Canker Sores?
The most common cause of a single mouth ulcer is injury, such as biting your cheek or scraping your gum with a toothbrush. For canker sores that keep coming back, the exact cause (the etiology, in textbook terms) still isn't fully understood. What we do know is which triggers make them more likely:
Accidental bites and burns. Examples include biting your cheek while chewing, a hot pizza on the roof of your mouth, or a crusty bread roll.
Sharp teeth, fillings or dental appliances. These include a chipped tooth, a broken filling, a denture that rubs, or a brace bracket.
Stress and poor sleep.
Hormonal changes. Many women notice ulcers at the same point in their menstrual cycle.
Your toothpaste. Many toothpastes contain a foaming agent called sodium lauryl sulfate (SLS), which can irritate the mouth lining in people prone to ulcers.
Food triggers. Citrus, tomato, vinegar, chilli, pineapple and very crunchy foods are the usual suspects.
Low iron, vitamin B12 or folate.
Stopping smoking. Ulcers sometimes flare in the first few weeks after quitting. It's annoying, but it's temporary and no reason to start again.
Medical conditions. These include coeliac disease, Crohn's disease, ulcerative colitis, Behçet's disease and conditions that lower your immunity.
Some medications. Examples include certain anti-inflammatories, a heart medication called nicorandil, and chemotherapy.
Look closely at number two. When a patient tells us the ulcer always turns up in the same spot, the first thing we do is run a finger along the nearby teeth. More often than not, there's a rough edge rubbing the same patch of cheek or tongue. No gel will fix that. Smoothing the tooth will.
Why do I keep getting mouth ulcers?
If you get three or more mouth ulcers a year, there's usually a pattern worth finding. The most common culprits we see are the same sharp edge, an SLS toothpaste, stress and a nutritional gap.
It's worth ruling out coeliac disease, which affects around 1 in 70 Australians and often goes undiagnosed. Recurring ulcers can be one of its few visible signs. Your GP can arrange a simple blood test for coeliac disease, iron, B12 and folate.
Can stress cause ulcers in your mouth?
Yes. Stress is one of the best-documented triggers for canker sores in the mouth, although the exact mechanism is still being studied.
We tend to see more ulcer-related visits from teenagers and uni students across Campbelltown and the Macarthur region around HSC and exam time. Poor sleep, skipped meals and lip or cheek biting under pressure all add up.
Where Do Mouth Ulcers Appear?
Mouth ulcers usually form on the soft, moveable lining of the mouth. That includes the inside of the cheeks and lips, the tongue, the floor of the mouth and the soft palate at the back. Where it sits can tell you a lot about what it is.
Mouth ulcer on the gum (canker sore inside gums)
Aphthous ulcers are less common on the firm gum tissue that hugs your teeth, and more common where the gum meets the cheek. A mouth ulcer on the gum is often caused by brushing too hard or by a sharp food edge.
Here's the catch. A sore, raised spot on the gum next to a tender tooth may not be an ulcer at all. It could be a gum boil from a dental abscess, which needs treatment rather than a gel. We explain the difference in our guide to dental abscess symptoms and treatment.
Canker sore on or under the tongue
The sides and underside of the tongue are common spots. A canker sore on the tongue often hurts more than others because the tongue never stops moving.
Ulcers under the tongue can make talking and swallowing uncomfortable. A sore on the side of the tongue that doesn't heal should always be checked (see the red flags below).
Mouth ulcer on the roof of the mouth
On the hard front part of the palate, a sore is usually a burn from hot food or drink. Aphthous ulcers are more likely on the soft palate at the back. Burns typically settle within a week.
Inside the lip or at the corner of the mouth
A canker sore inside the lip is very common, often after an accidental bite. Sores at the corner of the mouth or on the outside of the lips are a different story. These are usually cold sores or angular cheilitis (cracked, inflamed corners), which the table below helps you tell apart.
Canker sore in the throat
Ulcers can form near the tonsils. If you also have a fever, a sore throat and swollen glands, see your GP, because a viral or bacterial infection may be involved.
Mouth Ulcer vs Cold Sore vs Other Mouth Sores
Mouth sores aren't all the same. This table covers the ones people most often mix up:
Sore | Where it appears | What it looks like | Contagious? | What usually helps |
Mouth ulcer / canker sore | Inside cheeks, lips, tongue, soft palate | Round white or yellow crater with a red rim | No | Salt water, protective gels, avoiding triggers |
Cold sore (herpes simplex virus) | Outside the lips, around the mouth | Tingle first, then a cluster of blisters that crust | Yes | Antiviral cream from the pharmacist, started early |
Oral thrush | Tongue, cheeks, palate | Creamy white patches that wipe off, leaving red skin | Rarely | Antifungal treatment from a GP or pharmacist |
Angular cheilitis | Corners of the mouth | Red, cracked, sore corners | No | Treating fungal infection, checking denture fit and iron levels |
Hand, foot and mouth disease | Mouth, plus hands and feet | Small ulcers plus a spotty rash, usually in young kids | Yes | Rest, fluids, pain relief. Usually settles in 7–10 days |
The short version: if it's inside your mouth and there's no blistering, it's most likely a canker sore. If it's on the outside of your lip and tingled first, think cold sore.
How Long Does a Mouth Ulcer Take to Heal?
Most minor mouth ulcers heal within 7 to 14 days. The Better Health Channel advises seeing your dentist or doctor if an ulcer hasn't cleared after 14 days, or if you get them often. Better Health Channel
Timeframe | What's normal | What to do |
Days 1–3 | Tingling or burning, then a small painful sore appears | Start salt water rinses and avoid triggers |
Days 4–7 | Pain peaks, then starts easing. The centre may look yellow-grey (this is normal healing tissue, not pus) | Keep going with home care |
Days 7–14 | Ulcer shrinks and pain fades | Nothing more needed if it's healing |
Days 14–21 | Ulcer still there | Book a check-up |
Day 21 and beyond | Not healed | Don't wait. Get it examined promptly |
That yellow centre catches a lot of people out. It's a normal layer of healing tissue. An infected mouth ulcer looks different: it gets more painful after day four or five instead of less, and it may be swollen, weep pus, or come with a fever.
How to Get Rid of a Mouth Ulcer at Home
You can't make a mouth ulcer vanish overnight, but you can cut the pain and give it the best chance to heal. These mouth ulcer home remedies are the ones we recommend most:
Rinse with warm salt water. Use half a teaspoon of salt in a cup of warm water. Swish for 30 seconds, three or four times a day. It stings briefly, then soothes.
Switch to an SLS-free toothpaste. Check the ingredients list. More on why below.
Use a soft toothbrush and go gently around the sore spot.
Avoid your triggers for a few days. That means acidic, spicy, salty, crunchy and very hot food and drink.
Numb it before meals with a pharmacy gel so you can eat comfortably.
Try an alcohol-free antiseptic mouthwash for a week or so. Chlorhexidine can reduce the risk of infection, but it may cause temporary brown staining, so don't use it long term.
Stay hydrated. A dry mouth makes ulcers feel worse. If your mouth is often dry, our article on what causes dry mouth may help.
Take paracetamol or ibuprofen as directed on the pack if the pain is affecting sleep or eating.
Can you cure a mouth ulcer in one day?
No. Anything promising to cure a mouth ulcer in one day is overselling. Your mouth lining needs time to rebuild. What you can do in a day is make it much less painful, and that's usually what people really want.
Mouth ulcer gels, creams and patches: what's in them?
Pharmacy shelves are full of mouth ulcer products. It's easier to choose once you know the active ingredients:
Active ingredient | What it does | Good to know |
Benzydamine (spray, rinse or gel) | Anti-inflammatory and numbing | Useful before meals. Can sting at first |
Lignocaine (gel) | Local anaesthetic, numbs the area | Short-acting, so apply just before eating |
Choline salicylate (gel) | Pain relief, related to aspirin | Not for anyone under 16 (see below) |
Antiseptic (chlorhexidine, cetalkonium) | Lowers the risk of infection | Doesn't speed healing on its own |
Protective pastes and patches | Forms a barrier over the ulcer | Handy for ulcers that get rubbed or bumped |
Salicylate gels deserve a caution. UK regulators ruled in 2009 that oral salicylate gels shouldn't be used in anyone under 16, after a suspected case of Reye's syndrome in a toddler. Australian doctors have also reported significant salicylate poisoning in children from teething gels containing choline salicylate. For kids and teens, ask your pharmacist for a lignocaine or benzydamine product instead.
What about natural remedies for mouth ulcers? Honey has some small studies behind it and is gentle to try. Please don't hold an aspirin tablet against the ulcer. It causes a chemical burn and makes things worse.
How Dentists Treat Stubborn or Recurring Mouth Ulcers
Dental treatment for mouth ulcers focuses on finding and removing the cause, not just numbing the sore. Here's what that usually involves:
Smoothing sharp edges on teeth or replacing broken fillings. A rough filling can often be swapped for a smooth tooth-coloured white filling.
Adjusting dentures that rub. If yours no longer fit well, we can look at relining or replacing your dentures.
Orthodontic wax or bracket adjustments for brace sores.
Prescription options for severe or frequent ulcers, such as a steroid mouthwash or paste.
Blood test referrals through your GP to check for iron, B12 or folate deficiency and coeliac disease.
Biopsy referral for any ulcer that looks unusual or won't heal.
One pattern we see often is patients who've spent weeks on pharmacy gels for an ulcer caused by a jagged tooth. A few minutes of smoothing in the chair and the ulcer finally gets the chance to heal.
If an ulcer has lingered past two weeks or keeps coming back, book a dental check-up in Campbelltown and we'll look for the cause. If the pain is severe or comes with facial swelling, our emergency dental appointments can get you seen sooner.
How to Prevent Mouth Ulcers
You can't prevent every ulcer, but these habits cut down how often they appear:
Use an SLS-free toothpaste if you're prone to ulcers
Brush with a soft brush, gently
Get sharp teeth, chipped fillings and rubbing dentures fixed early
Eat slowly and chew carefully, especially when tired or stressed
Keep a simple diary of when ulcers appear to spot food or cycle triggers
Eat plenty of iron, B12 and folate sources, such as leafy greens, legumes, eggs, meat and fish (our guide to nutrition and your oral health covers more)
Keep up regular dental visits so problems get caught early
The toothpaste tip has better evidence than most people realise. A 2019 systematic review found that SLS-free toothpaste reduced ulcer duration, the number of episodes and ulcer pain compared with SLS toothpaste. The evidence isn't strong yet: the studies were small, with 132 patients in total, and none was rated low risk of bias. Still, it's cheap, low-risk and worth trying for a month or two. National Elf ServiceNational Elf Service
When Should You See a Dentist About a Mouth Ulcer?
See a dentist if a mouth ulcer hasn't healed within two weeks, and don't put it off if it's still there at three. Book in sooner if you notice any of these:
An ulcer larger than 1 cm, or very deep
An ulcer that's painless, feels hard, or has raised, rolled edges
Red or white patches that don't rub off
A lump, or numbness in your lip, tongue or chin
Ulcers that keep coming back, three or more a year
Ulcers with fever, spreading redness or swelling
Pain you can't manage with pharmacy pain relief
People often ask about the difference between a mouth ulcer and cancer. A typical canker sore is painful early, soft around the edges and heals within two weeks. Early mouth cancer is often painless, feels firm and doesn't heal.
Most ulcers are harmless, but the only way to be sure about a persistent one is to have it examined. Our article on early detection of mouth cancer explains what we look for, and Cancer Council Australia has more on risk factors.
Mouth Ulcers in Children and During Pregnancy
Children. Kids get mouth ulcers from bumps, bites and new teeth, and also from viral infections. Hand, foot and mouth disease is common in childcare and brings mouth ulcers plus a rash.
Toddlers can also get a first herpes infection inside the mouth. It causes many painful ulcers and swollen gums, often with a fever, and it needs a GP visit. Avoid salicylate gels for anyone under 16.
If your child's ulcers keep returning, our children's dentistry team can help find the cause. Eligible families may be covered under the Child Dental Benefits Schedule.
Pregnancy. Hormonal changes can make mouth ulcers more frequent in pregnancy. Salt water rinses are safe. Check with your pharmacist or GP before using medicated gels, especially salicylate products.
Getting a Mouth Ulcer Checked
Most mouth ulcers are a short-lived nuisance you can manage at home. But if one hasn't healed in two weeks, keeps coming back in the same spot, or just doesn't look right to you, it's worth having it looked at. We see mouth ulcers regularly at Bradbury Dental Surgery, and a quick exam can usually tell you whether it's a sharp tooth, a trigger to avoid, or something that needs a closer look.
Call us on (02) 4628 2151 or book online.
Frequently Asked Questions
Are canker sores contagious?
No. Canker sores aren't caused by a virus and can't be passed on by kissing or sharing drinks. Cold sores, which appear on the outside of the lips, are contagious.
What's the difference between a canker sore and a mouth ulcer?
There isn't one. "Canker sore" is the American term and "mouth ulcer" is the one most Australians use. Both describe the same aphthous ulcer.
Is an infected mouth ulcer serious?
It's uncommon, but it does need attention. Signs include pain that gets worse after day four or five, swelling, pus or a fever. See your dentist or GP if you notice these.
Can toothpaste cause mouth ulcers?
It can contribute in people who are prone to them. Research suggests toothpastes containing sodium lauryl sulfate (SLS) may trigger more frequent ulcers in some people, so switching to an SLS-free brand is worth trying.
Should you pop a mouth ulcer?
No. A mouth ulcer isn't a pimple, and there's nothing to drain. Picking or squeezing it damages the tissue further and slows healing.
