Mouth Ulcers: How to Recognise Them, the 3 Types, Causes, Relief and When to See a Doctor

Mouth ulcers are very common. Most heal on their own within one to two weeks, and they are not contagious. Hong Kong's Department of Health advises seeing a doctor or dentist if an ulcer lasts more than three weeks or keeps coming back. This guide covers what they look like, the three types, causes, relief, and how to tell them apart from cold sores and hand, foot and mouth disease.

What is a mouth ulcer and how do you recognise one?

The Department of Health describes aphthous ulcers as a kind of mouth ulcer that is not infectious but tends to recur. They start as small oval or round blisters; after a day the edge turns red and the centre yellowish or greyish-white. Common places are the inside of the cheeks and lips, the tongue and under the tongue.

Three types of mouth ulcer

Type	Size	Healing	Share of cases
Minor (most common)	Usually under 5 mm, 1–6 at a time	Within two weeks, no scar	Over 70%
Major	Over 10 mm	5–10 weeks, usually scars	About 10%
Herpetiform	2–3 mm, can be dozens to 100 at once	1–2 weeks	Uncommon

Source: StatPearls: Aphthous Ulcers (updated 2023). Recurrent mouth ulcers affect up to 25% of people worldwide, and up to 46% of patients report a family history.

Why do mouth ulcers happen?

The exact cause isn't fully understood; usually several factors combine. Common triggers listed by Hong Kong's Department of Health and the UK's NHS include:

• injury to the mouth: biting your cheek, brushing too hard, badly fitting dentures, braces, a sharp broken tooth or a rough filling;
• stress and tiredness;
• hormonal changes, such as periods or pregnancy;
• low levels of nutrients: vitamin B12, folic acid and iron (the NHS also lists zinc and vitamin D);
• gut conditions such as coeliac disease or inflammatory bowel disease;
• stopping smoking, and some medicines.

Sources: Department of Health oral health education, NHS: Mouth ulcers.

Can toothpaste cause ulcers? For some people, possibly. The NHS advises avoiding toothpaste containing sodium lauryl sulphate (SLS) while you have ulcers. A 2019 systematic review (four small trials, 132 people) found that people using SLS-free toothpaste had fewer ulcers, shorter episodes and less pain, though the authors said the evidence is limited and more trials are needed (National Elf Service summary). If you get ulcers often, try an SLS-free toothpaste for a few months and see whether it helps. For choosing a toothpaste, see how to choose a toothpaste.

How to ease a mouth ulcer

What you can do yourself

• use a soft-bristled toothbrush and brush around the ulcer, but don't stop brushing because it hurts;
• avoid very spicy, salty, acidic, hot or rough food (such as toast or crisps) for now;
• choose soft food such as congee, steamed egg or tofu, and drink cool drinks through a straw;
• rinse with warm salt water: the Mayo Clinic suggests about half a teaspoon of salt in a cup (about 240 ml) of warm water. Don't rub salt directly on the ulcer; it only makes it hurt more.

What a pharmacist or doctor can offer

The NHS lists pharmacy options including antimicrobial mouthwash, painkilling gels, corticosteroid lozenges and saline mouthwash. A doctor can also prescribe steroid or anaesthetic mouth rinses, and if blood tests show low folate, B vitamins or zinc, these can be treated (Mayo Clinic).

If an ulcer is caused by a broken tooth, a sharp filling edge or badly fitting dentures, see a dentist to fix the cause, or it will keep coming back.

When to see a doctor or dentist

The Department of Health advises a check-up if an ulcer lasts more than three weeks or new ulcers keep appearing. The NHS also says to seek help if an ulcer:

• has not healed after three weeks;
• is different from ones you've had before, for example bigger than usual or near the back of your throat;
• bleeds, or becomes more painful and red;
• comes with a fever, or makes it too painful to eat.

The NHS notes that a long-lasting mouth ulcer is sometimes a sign of mouth cancer, so it's advisable to get it checked. Warning signs listed by the Department of Health include an ulcer that won't heal, a lump with a sunken centre or uneven surface, unexplained bleeding in the mouth, sudden facial numbness, teeth loosening for no reason and swollen neck glands. Heavy smoking, drinking and chewing betel nut are major risk factors (Department of Health: oral cancer, Chinese).

Mouth ulcer, cold sore or hand, foot and mouth disease?

	Mouth ulcer	Cold sore	Hand, foot and mouth disease
Where	Inside the mouth (cheeks, inner lips, tongue)	Edge of the lips or nostrils	Mouth sores plus a rash on the palms and soles
Cause	Not fully known; several triggers	Herpes simplex virus type 1	Enteroviruses (common in children)
Contagious?	No	Yes	Yes
Other	Heals in 1–2 weeks	Blisters, then a scab; about 10 days	Usually with fever; resolves in 7–10 days

For cold sores, see what to do about cold sores. For hand, foot and mouth disease, see the Centre for Health Protection; in Hong Kong it usually peaks from May to July, with a possible second peak from October to December.

Preventing ulcers from coming back

• use a soft toothbrush and don't brush too hard;
• notice whether certain foods seem to trigger them, such as chocolate, nuts, or spicy or acidic food;
• try an SLS-free toothpaste;
• keep regular sleep and manage stress;
• if you get them often, ask a doctor about blood tests (for example iron, B12 and folate);
• see a dentist regularly to deal with sharp tooth edges, badly fitting dentures or rubbing from braces.

Frequently asked questions

Are mouth ulcers contagious?

No. The Department of Health and the NHS both say mouth ulcers are not contagious. Contagious blisters on the outside of the lips are more likely to be cold sores.

How long does a mouth ulcer take to heal?

Usually 7 to 14 days. Major ulcers can take several weeks. See a doctor or dentist if one hasn't healed after three weeks.

Do frequent ulcers mean too much "heatiness"?

Modern medicine links recurrent ulcers to mouth injury, stress, hormones, low nutrient levels and gut conditions. If you get them often, see a doctor to rule out deficiencies or other health problems.

Will rubbing salt on an ulcer help it heal faster?

No. Salt applied directly irritates the wound. Rinse with diluted warm salt water instead.

What about an ulcer on the tongue?

Treat it the same way: avoid irritating food, keep the mouth clean and use a painkilling gel. The tongue moves constantly, so it hurts more. If a tongue ulcer lasts more than three weeks, feels hard or bleeds, get it checked.

Sources

• Department of Health oral health education: aphthous ulcers
• Department of Health oral health education: oral cancer (Chinese)
• NHS: Mouth ulcers (reviewed 2024-03-11)
• Mayo Clinic: Canker sore (2026-08-26)
• StatPearls: Aphthous Ulcers (updated 2023-11-13)
• National Elf Service: SLS toothpaste and recurrent ulcers (2019-04-17)
• Centre for Health Protection: hand, foot and mouth disease

This article is general health information and does not replace diagnosis or treatment advice from a doctor or dentist.