Canker sores: causes, how long they last and what helps

In short

• A canker sore is a small round or oval sore inside the mouth, often white or gray in the center with a red border. It can hurt a lot for its size.
• Canker sores are not contagious and are not the same as cold sores, which are caused by the herpes simplex virus.
• The exact cause is unknown. Mouth injuries, stress, tiredness, hormonal changes, some foods and low vitamin or iron levels are common triggers.
• Most heal within one to two weeks without treatment. Avoiding irritating foods and using a pharmacy mouth rinse or gel can ease the pain.
• See a dentist or primary care provider if a sore lasts more than three weeks, keeps coming back, looks different or bleeds.

What canker sores are

Canker sores, also called mouth ulcers or aphthous ulcers, appear on the inner cheeks, inside the lips, on or under the tongue, or at the back of the throat. You can have one or several at once.

MedlinePlus describes them as small, round sores with a red border and a gray center that can be quite painful. They are not contagious, so you cannot catch one from kissing or sharing a cup.

Cold sores are different. They are caused by herpes simplex, usually show up on or around the lips, and often start with tingling, itching or burning before the blister forms.

What you may notice

• A painful spot that stings when you eat or drink anything hot, salty, spicy or acidic.
• A round or oval sore, white or gray, edged in red.
• One sore or a few, mostly on the soft lining of the mouth.
• Healing on its own, usually over about one to two weeks.

These details describe an ordinary canker sore. A large sore near the back of the throat, a sore that bleeds, or sores with fever are not the typical picture, and the NHS lists them as reasons to get checked.

Causes and triggers

The American Dental Association says the exact cause is unknown. Experts suspect problems with the immune system, bacteria or viruses, and injury to the soft tissues of the mouth can set off a sore.

• Biting your cheek, ill-fitting dentures or braces, rough fillings, a sharp tooth, or brushing too hard.
• Stress and tiredness.
• Hormonal changes, including those linked to menstrual periods and pregnancy.
• Food intolerances or allergies, and foods that irritate the mouth.
• Low levels of iron, zinc, folic acid, B vitamins or vitamin D.
• Certain medicines, family history, and for some people stopping smoking.
• Toothpaste that contains sodium lauryl sulphate, which the NHS lists as an irritant for some people.

The NHS adds that several sores at once can point to another condition, such as hand, foot and mouth disease, oral lichen planus, Crohn's disease, coeliac disease or a weakened immune system. That is one reason frequent or widespread sores should be examined.

What helps at home

1. Avoid irritants.Skip spicy, salty, acidic and rough or crunchy foods, and very hot or acidic drinks, until the sore heals.
2. Be gentle.Use a soft toothbrush, drink cool drinks through a straw and choose softer foods.
3. Rinse with salt water.Dissolve a little salt in warm water, rinse, then spit it out.
4. Ask a pharmacist.They can suggest an antimicrobial mouthwash, painkillers, or corticosteroid lozenges. Over-the-counter numbing products and rinses may give temporary relief. Follow the package directions, and check with a pharmacist before using them for a child.
5. Check your toothpaste.If sores keep returning, try one without sodium lauryl sulphate.

The ADA also notes that some oral bandages can help protect a sore and reduce secondary infection.

When to see a dentist or doctor

• The sore lasts longer than three weeks. MedlinePlus gives the same limit for any mouth ulcer.
• It looks different from your usual sores, is larger than usual or is near the back of the throat.
• It bleeds, or becomes more painful and red, which may signal an infection.
• You have sores elsewhere, such as on the skin or genitals, or painful, red or swollen joints.
• Sores keep coming back or new symptoms appear.

Start with your dentist or your primary care provider. A sore that never heals can occasionally be an early sign of mouth cancer, which is why the three-week rule matters.

Red flags, call 911 now: swelling of the lips, tongue, face or throat; trouble breathing or swallowing; fainting or sudden confusion. These are not features of a canker sore itself. They can signal a serious allergic reaction, for example to a food or medicine, and they are an emergency.

How a clinician checks and treats it

A dentist or doctor usually recognizes a canker sore by looking at it and asking about your history. They check for local causes such as a sharp tooth or an ill-fitting denture, and look for other conditions if sores are frequent or widespread. Your clinician decides whether any tests are needed.

For severe, long-lasting or infected sores, the NHS says a GP or dentist may prescribe steroids as a mouth spray, dissolving tablets or tablets dissolved in water, stronger painkillers, or mouthwashes that kill germs. They use these only as prescribed. If a food, medicine or habit seems to trigger the sores, they will help you sort that out.

Preventing the next one

• Learn your triggers and avoid them, such as sharp or rough foods, hot or acidic drinks and irritating toothpaste.
• Brush at least twice a day, floss daily and get regular dental check-ups.
• Keep to a healthy, balanced diet.
• Ask a dentist to smooth a sharp tooth or adjust a denture or brace that keeps rubbing.

Questions and answers

Sources

1. NHS — Mouth ulcers
2. American Dental Association (MouthHealthy) — Canker Sores
3. MedlinePlus — Canker Sores
4. MedlinePlus — Mouth ulcers

Sources checked on 28 October 2026.