Mouth Ulcers: Why They Appear and How to Ease Them

A mouth ulcer (also called an aphthous ulcer or canker sore) is an open, painful sore inside the mouth, with a whitish or yellowish centre and an inflamed red edge. It is not cancerous, it is not contagious and it has nothing to do with cold sores. It almost always goes away without treatment; what you can do is reduce the pain while it heals and avoid what triggers it.

What they look like and how long they last

They appear on the inside of the cheeks and lips, on the tongue, on the floor of the mouth or on the soft palate. They are usually felt first as a tingling or burning and, a day or two later, the ulcer forms. The three usual types are told apart by size and duration:

Type	Size	Approximate duration	Scar
Minor (most cases)	Under 1 cm	4 to 14 days	No
Major (around 10%)	Over 1 cm	More than 6 weeks	Possible
Herpetiform (around 10%)	2 to 3 mm, in clusters	Under 30 days	Uncommon

With minor ulcers, the pain usually eases in 7 to 10 days and full healing takes between one and three weeks. Major ulcers do need a medical assessment.

Why they appear

The exact cause is not known. What has been described is a list of triggers that can make them more likely:

• Small injuries in the mouth: an accidental bite, brushing too hard, a filling or a brace that rubs.
• Emotional stress.
• Hormonal changes. They are somewhat more common in women.
• Low levels of iron, folic acid or vitamin B12.
• Food allergies or intolerances in some people.
• Tobacco: the US National Institute of Dental and Craniofacial Research (NIDCR) lists it among the possible factors.

When mouth ulcers keep coming back, it is also worth ruling out underlying causes: some medicines and digestive diseases such as Crohn’s disease or coeliac disease can show up this way. That assessment is a job for a doctor.

How to ease them at home

Since they heal on their own, the aim is to make eating and talking more bearable. What helps:

1. Avoid irritants: spicy, acidic, very salty or hard foods, and very hot drinks.
2. Rinse with salt water or an alcohol-free mouthwash, which doesn’t sting on the ulcer.
3. Use an over-the-counter gel or product to soothe the pain, following the leaflet. Local anaesthetics such as lidocaine or benzocaine numb the area for a few minutes, but they give only short-lived relief and don’t speed up healing.
4. Keep up your oral hygiene with a soft toothbrush. An antiseptic such as chlorhexidine can help prevent the ulcer becoming infected, although it doesn’t shorten the process on its own.
5. Don’t touch the ulcer with your tongue or fingers.

In severe cases a doctor or dentist may prescribe topical corticosteroids or, more rarely, treatments taken by mouth. They are not products to use on your own over long periods.

What not to do

• Don’t confuse a mouth ulcer with a cold sore. Cold sores appear on the outer edge of the lip, as clusters of blisters, and they are contagious.
• Don’t take iron, folic acid or B12 supplements “just in case”: it only makes sense if a blood test shows a deficiency.

How to reduce flare-ups

There is no formula that guarantees they won’t come back, but these measures reduce the known triggers:

• Choose a soft-bristled toothbrush and don’t brush hard.
• Ask your dentist to check for sharp edges on teeth or braces that rub.
• Notice whether they appear after certain foods and, if so, avoid them for a few weeks to check.
• Eat a varied diet with iron, folate and B12. If you suspect a deficiency, ask for a blood test rather than taking supplements blindly.
• If you smoke, stopping is a sensible step for your mouth too.

Mouth ulcer or cold sore: how to tell them apart

They are often confused, but they are different problems. A mouth ulcer appears inside the mouth, is a single sore or a few, and is not contagious. A cold sore, on the other hand, usually appears on the outer edge of the lip or around the mouth, starts as clusters of blisters, crusts over and does spread through contact. The treatment and precautions are different, so identifying which one you have avoids mistakes, such as sharing a glass or kissing someone during an active cold sore.

A practical point about the pain

Mouth ulcer pain gets worse when you eat, drink and talk because the ulcer exposes nerve endings. For the first few days it helps to eat soft, lukewarm food, drink through a straw if the ulcer is on one side, and avoid nuts or toast, which rub. As the discomfort eases, you can gradually go back to your usual diet.

When to see a doctor

Book an appointment with your doctor or dentist if:

• The ulcer lasts more than two weeks without improving.
• You get ulcers more than two or three times a year or new ones appear before the old ones heal.
• They are very large or so painful that you can’t eat or drink normally.
• They come with fever, diarrhoea, headache or a skin rash.
• You notice an ulcer with hard edges, that bleeds or that won’t heal, especially if you smoke or drink alcohol often.

If you have difficulty swallowing, facial swelling or signs of infection (pus, a persistent bad smell), get help without waiting.

This article is for information only and does not replace an assessment by a healthcare professional. Sources consulted: NIDCR, Fever Blisters and Canker Sores · MedlinePlus, Canker sore · Recurrent Aphthous Stomatitis: A Review, PMC.