Canker sores are small, shallow ulcers that form inside the mouth — on the inside of the lips or cheeks, on the tongue, or at the base of the gums. They are painful but harmless, are not contagious, and almost always heal on their own within one to two weeks. Their medical name is aphthous ulcers, and they are one of the most common mouth problems people get.

This page is general information to help you understand canker sores and ease them safely. It is not a diagnosis. If a sore lasts a long time, keeps coming back, or something doesn't feel right, see a doctor or dentist.

What is a canker sore, and how is it different from a cold sore?

A canker sore is an ulcer that forms inside the mouth, while a cold sore is a fluid-filled blister that forms outside the mouth, usually on or around the lips. This difference matters, because the two are caused by completely different things and are treated differently.

Canker sores are typically round or oval, with a white or yellowish centre and a red border. They appear on the soft tissues inside the mouth and are not caused by a virus and are not contagious — you cannot pass them to anyone else.

Cold sores, by contrast, are caused by the herpes simplex virus, are contagious, and tend to appear on the outer edge of the lips, sometimes starting with a tingle before a cluster of blisters forms. If your sore is on the outside of your lip and blistering, it is more likely a cold sore than a canker sore.

What causes canker sores?

The exact cause of canker sores isn't fully understood, but they are often set off by a trigger — most commonly a minor injury to the inside of the mouth. Knowing your own triggers is the most useful step in reducing them.

Common triggers include:

Minor mouth injury — biting the inside of your cheek, a rough or sharp food, a poking filling, or ill-fitting dentures or braces.
Stress and tiredness, which seem to make many people more prone to them.
Hormonal changes, for example around a menstrual period.
Certain foods, such as acidic, spicy, or rough foods in some people.
Toothpaste containing sodium lauryl sulfate (SLS), a foaming agent that can irritate some mouths.
Nutritional shortfalls, including low iron, zinc, or vitamin B12.

Some people get them repeatedly with no obvious cause — this is called recurrent aphthous stomatitis. If you're looking at whether a deficiency is playing a part, it's worth reading our guides on zinc and vitamin D, though a blood test from your doctor is the only way to know for sure.

What are the stages of a canker sore, and how long does it last?

Most canker sores follow a recognisable pattern and clear up within one to two weeks without any treatment. Larger sores can occasionally take longer.

A typical canker sore goes through these stages:

Tingling or burning — a day or so before anything is visible, you may feel a slight burning or prickling at the spot.
A small red bump or sore patch appears.
The open ulcer forms, with a white or yellow centre and a red rim. This is usually the most painful stage.
Healing, as the soreness eases and the tissue closes over, usually leaving no scar.

Most ordinary (minor) canker sores heal within 7 to 14 days. Larger ones can take several weeks. Pain often settles well before the sore is fully healed.

What can I feel at the back of my throat or further down?

Canker sores most often form on the lips, cheeks, and tongue, but they can also appear further back, near the soft tissue at the back of the throat, which can make swallowing uncomfortable. A sore in this area can feel like something is stuck.

If you keep feeling something at the very back of your throat, bear in mind that not everything there is an ulcer — tonsil stones can cause a similar "something stuck" sensation, and a sore throat from infection can too. If you're unsure what you're feeling, or it doesn't settle, it's worth getting it looked at.

How do I get rid of a canker sore safely?

You can't make a canker sore vanish overnight, but you can ease the pain and avoid irritating it while it heals on its own. The mainstays are gentle and low-risk.

Helpful self-care includes:

Rinse with warm salt water (about half a teaspoon of salt in a glass of warm water), swished around the mouth and spat out. This soothes and keeps the area clean.
Use a soft toothbrush and brush gently to avoid catching the sore.
Eat softer, cooler foods and drink cool liquids — using a straw can help keep drinks away from a sore.
Avoid things that sting it — spicy, salty, acidic, or rough foods, and very hot drinks, while it heals.

A pharmacist is a good first port of call for over-the-counter options. These can include antimicrobial mouthwashes, pain-relieving gels or sprays that numb the area, and corticosteroid lozenges designed for mouth ulcers. Ask the pharmacist which is suitable for you, and follow the instructions on the pack.

What should I not do?

Do not ignore a sore that simply won't heal, and don't try to burn or scrape an ulcer away — this damages healthy tissue and slows healing. Avoid applying undiluted strong substances directly to the sore in the hope of "drying it out," as this can make the pain and irritation worse.

Be wary of strong home remedies you may read about online. Neat apple cider vinegar, for example, is acidic and can sting and irritate an open ulcer — read our apple cider vinegar guide for a balanced look at what it can and can't do. If a particular toothpaste seems to bring on ulcers, switching to one without sodium lauryl sulfate is a reasonable thing to try.

When should I see a doctor or dentist?

See a GP or dentist if a mouth ulcer lasts longer than three weeks. A single ulcer that does not heal within three weeks is a recognised red flag and should always be checked, as on rare occasions a persistent ulcer can be a sign of something more serious, including mouth cancer.

You should also get advice if you have:

Ulcers that keep coming back or that you get very frequently
An ulcer that is unusually large or spreading
Severe pain that isn't controlled by pharmacy treatments
Signs of infection, such as increasing redness, swelling, or pus
Ulcers alongside other symptoms, such as a high temperature or swollen glands that don't settle

A persistent ulcer doesn't usually mean anything sinister, but it does mean it's worth a professional look. This page is background information and not a substitute for advice from a healthcare professional.

This article is for general information only and is not a substitute for professional medical advice. Always consult a qualified healthcare provider about your individual situation.