Mouth Ulcers: Causes, Treatment and When to Worry

What Are Mouth Ulcers?

Mouth ulcers - you've almost certainly had one. These small, shallow sores pop up on the soft tissue inside your mouth: inside the cheek, on the tongue, along the gum line. They're round or oval, usually white or yellow in the centre with a red halo. And they sting like mad when you eat anything acidic.

Clinically they're called aphthous ulcers, and they're common. For most people it's a minor nuisance that clears up on its own in seven to fourteen days. The soreness peaks around day two or three, then fades. A smaller group develops larger ulcers - over 1cm, deeper, and slower to heal. Those are the ones that can leave scars.

Patients often worry that a persistent ulcer might be oral cancer. In the great majority of cases it isn't. The key difference is behaviour: a cancerous lesion usually doesn't hurt much, grows slowly, and doesn't heal after three weeks. A plain mouth ulcer hurts from the start and starts shrinking within ten days. If it's still there after two weeks, get it checked.

There's also a distinction between minor and major ulcers, plus a less common type that comes in clusters - herpetiform - which looks like herpes but isn't. They can pop up after a cheek bite, during stress, or with certain foods. Toothpastes containing sodium lauryl sulphate appear to be a trigger for some people, and swapping to an SLS-free paste is a simple thing to test.

Bottom line: mouth ulcers are painful but predictable. They're your body's way of saying something's off - a minor injury, a nutritional dip, or just bad luck. Most resolve without any treatment. The trick is knowing when to leave them alone and when to call a dentist.

Common Triggers of Mouth Ulcers

Stress is the trigger patients mention most often. A run of bad sleep, an exam, a big life change - and a few days later a sore appears. The body's immune response shifts, and the lining of the mouth takes the hit.

Physical damage is another common one. A sharp chip of crisps, a rough filling, biting the inside of your cheek. That small tear normally heals, but in people prone to mouth ulcers it can turn into a full canker sore instead. Even aggressive brushing with a hard-bristled toothbrush can do it. Switch to a soft head and see if the frequency drops. If a rough edge on a tooth or restoration keeps catching the same spot, a dentist can smooth it, and a professional teeth cleaning appointment is a good opportunity to have that checked.

Food triggers vary person to person, but a few repeat offenders stand out. Acidic things - oranges, lemons, tomatoes, pineapple - are high on the list. Spicy dishes, chocolate, coffee and nuts also get blamed. Keeping a food diary for a few weeks can help you spot your own pattern.

Toothpaste ingredients matter too. Sodium lauryl sulphate (SLS) - the agent that makes toothpaste foam - irritates some mouths. Switching to an SLS-free paste for a month is a low-cost experiment, and some people find their ulcers become less frequent.

Nutritional gaps play a part. Low levels of B12, folic acid or iron are linked to recurrent ulcers. It isn't a universal cause, but a simple blood test can rule it out, and where a deficiency is found and corrected the ulcers often quieten down.

Hormonal shifts matter too. Some women notice ulcers appearing just before their period. The drop in oestrogen may make the oral lining more vulnerable. Not much you can do about that cycle, but knowing when they're likely can help you prepare - keep a gel or patch on hand.

A persistently dry mouth also leaves the lining more fragile and slower to recover, so if you often wake with a parched mouth it's worth reading up on dry mouth symptoms and treatment.

Underlying health conditions can be the hidden driver. Crohn's disease, coeliac disease and Behçet's syndrome can all cause mouth ulcers as a symptom. If your ulcers are frequent, painful, and come with other gut or skin issues, mention it to your GP. They might be the clue that leads to a diagnosis.

Your best tool for identifying triggers is a simple diary. Jot down the date, what you ate, how stressed you felt, and whether the sore appeared a day or two later. After a month or two, patterns usually emerge.

Types of Aphthous Ulcers

Aphthous ulcers - the medical name for the common mouth ulcer - aren't all the same. They split into three distinct types, each with its own size, healing time and pattern of pain. Knowing which one you're dealing with helps you judge what to expect and when to act.

Minor aphthous ulcers

These are the ones most people get, and they are by far the most common type. They're small - usually under 5 mm across - and shallow, with a greyish-white centre and a red halo. You'll likely have one to five at a time. They heal on their own within 7-14 days, and scarring is rare. Pain is sharp for the first 2-3 days, then fades. Annoying, yes. Dangerous, no.

Major aphthous ulcers

Less common but far more troublesome. They're bigger - often 10 mm or more - and dig deeper into the lining of the mouth. That means healing takes considerably longer, commonly several weeks, and scarring is common. These ulcers can make eating, drinking, even talking genuinely painful. They're not a sign of something malignant by themselves, but they need a dentist's look if they don't start healing after two weeks.

Herpetiform ulcers

Despite the name, these aren't caused by the herpes virus. They look like tiny clusters of pinpoint ulcers - many at a time - that can merge into larger, irregular sores. Each individual spot is 1-2 mm. They heal in about 7-14 days, but the sheer number can make the whole mouth feel raw. Herpetiform ulcers are the rarest type, mostly seen in young adults.

Quick comparison of aphthous ulcer types

Type	Size	Healing time	Scarring	How often seen
Minor	Under 5 mm	7-14 days	Rare	Most common
Major	10 mm or more	Several weeks	Common	Uncommon
Herpetiform	1-2 mm (clusters)	7-14 days	Uncommon	Rare

Most recurrent mouth ulcers are minor and nothing to worry about.

How to Soothe Mouth Ulcers: Self-Care and Relief

When a mouth ulcer flares up, every sip of tea or bite of toast can feel like a small punishment. The good news? Most ulcers heal on their own within a week or two. But there are real things you can do to speed that along and take the edge off the pain.

Start with what you're eating. Sharp, acidic foods - think citrus fruits, tomatoes, vinegar-based dressings - are direct irritants. So is anything crunchy or rough-edged: crisps, crusty bread, nuts. Soft, bland foods for a few days are far kinder. Porridge, yoghurt, mashed potatoes, scrambled eggs. Cold drinks help numb the area, and using a straw can keep liquid away from the sore spot.

Salt water rinses are the go-to home remedy for a reason. Dissolve half a teaspoon of table salt in a cup of warm water, swish it gently around your mouth for about 30 seconds, then spit it out. Two to three times a day is plenty. It won't make the ulcer vanish overnight, but it keeps the area clean and helps settle inflammation. Don't overdo it - too much salt can irritate further.

For more direct relief, over-the-counter gels containing benzocaine or lidocaine are widely available at UK pharmacies. They form a protective layer over the ulcer and numb the pain for a short while, which is usually enough to get through a meal. There are also protective pastes that stick to the moist tissue and act as a physical barrier against food and air. Ask your pharmacist which is most suitable for you.

Some people find honey soothing, dabbed in a tiny amount directly onto the ulcer. It stings briefly, then settles. Aloe vera gel is used in a similar way. Pharmacies also sell hydrocortisone buccal tablets for more stubborn cases, but these are best used under a pharmacist's advice and only for short periods.

A quick word about painkillers. Paracetamol or ibuprofen can help if the ulcer is really interfering with sleep or eating. Chlorhexidine mouthwash cuts down bacterial load and helps prevent secondary infection, but it shouldn't be used for more than a few days at a time because prolonged use can stain teeth.

One common mistake is worth naming: don't poke at the ulcer, prod it with your tongue, or try to pop it like a spot. That only restarts the healing clock.

When to See a Dentist About a Mouth Ulcer

Most mouth ulcers clear up on their own within 10 to 14 days, with no dentist visit needed. But a small minority signal something else entirely - an infection, an autoimmune flare, or in rare cases something more serious. So when exactly should you pick up the phone?

• Lasts longer than three weeks. If a sore hasn't started shrinking after 14 days and is still raw past day 21, get it looked at. That's the clearest red flag.
• Gets bigger, not smaller. A typical aphthous ulcer tops out at about 5-10 mm. If it's pushing past a centimetre, especially if it's growing, that's worth a check.
• Spreading or multiplying. One ulcer is annoying. A crop of new ones appearing while old ones haven't healed can point to a viral cause or a systemic condition.
• Bleeds or feels numb. Ulcers normally sting or burn. Bleeding when you touch them, or numbness around the area, are not typical and need a professional look.
• Comes with fever, rash or swollen glands. A mouth ulcer that arrives with body-wide signs - temperature, sore throat, lumps in the neck - might be part of a bigger infection.
• Makes eating or drinking impossible. If pain stops you swallowing even sips of water, or you've lost weight because you can't eat, that's past the point of home remedies.

It's also worth being clear about what a mouth ulcer is not. A sore, puffy area around a back tooth is a different problem - see swollen gum behind a back tooth for that pattern. Flat discoloured patches rather than open sores are another picture again, covered in our guide to red spots on the gums. And where the gums bleed and recede alongside repeated soreness, the underlying issue may need gum disease treatment rather than ulcer care.

What should you expect at the appointment? The dentist will examine the ulcer and look for other lesions in your mouth. They'll ask about your medical history - conditions such as Crohn's disease, coeliac disease or recurrent herpes can all cause mouth ulcers. If the sore looks suspicious - irregular edges, mixed colour, firm to touch - they may refer you for a biopsy. Most of the time, though, it's a straightforward diagnosis of a complex aphthous ulcer or a traumatic ulcer, and treatment is a topical steroid paste or antimicrobial mouthwash to speed healing.

Three weeks is the safe cutoff. An ulcer that has not begun to heal by then should always be assessed rather than waited out.

Frequently Asked Questions

How long should a mouth ulcer take to heal?

A minor aphthous ulcer usually heals within 7 to 14 days without treatment. Larger, deeper ulcers can take several weeks. If an ulcer has not started shrinking by around two weeks, and is still present at three weeks, book an appointment.

What is the difference between a mouth ulcer and a cold sore?

Mouth ulcers form on the soft tissue inside the mouth - the cheeks, tongue, inner lip and gum line - and are not contagious. Cold sores are caused by the herpes simplex virus, appear on or around the outside of the lips, usually begin with a tingling sensation and blister before crusting over, and are contagious.

Can stress really cause mouth ulcers?

Stress is one of the most commonly reported triggers. It doesn't create the ulcer directly, but periods of poor sleep, illness and pressure alter the immune response, and the lining of the mouth is often where that shows up first.

Are mouth ulcers a sign of a vitamin deficiency?

They can be. Low iron, vitamin B12 and folate are all associated with recurrent ulcers. It is not the cause in every case, but if your ulcers keep returning, a blood test through your GP is a simple way to rule it out.

Should I use mouthwash on a mouth ulcer?

An antimicrobial mouthwash such as chlorhexidine can help keep the area clean and reduce the risk of secondary infection, but it is intended for short-term use only, as prolonged use can stain teeth. Strong alcohol-based mouthwashes tend to sting and are best avoided while an ulcer is active.

When is a mouth ulcer serious?

Seek professional assessment if an ulcer lasts more than three weeks, keeps growing, is painless, bleeds, feels firm or numb, or comes with fever and swollen glands. Painless ulcers that do not heal are the pattern that most warrants prompt examination.