A canker sore is a shallow sore shaped like a crater (ulcer) on your tongue or on the inside of your lip or cheek. Canker sores have a red border and a white or yellow centre. They may be painful and can make it hard to talk and eat. You may have one or more than one canker sore at a time. Unlike cold sores, you cannot spread canker sores to other people.

Anyone can get a canker sore but women, teenagers, and young adults have them more often. People usually get their first canker sore between the ages of 10 and 40. Most people have canker sores at some time in their lives, and some people have them regularly.

The cause of canker sores is unknown, but they tend to run in families. Canker sores may also develop when you:

• Are stressed or tired.
• Have your menstrual cycle, if you are a woman.
• Hurt your mouth, such as biting your lip.
• Have braces on your teeth.
• Have food allergies. Eating foods that you are allergic to may cause you to get a canker sore.
• Eat or drink food or juice that has a lot of acid, such as orange juice.
• Do not get enough vitamins or minerals in your diet, such as iron.

Canker sores usually begin with a burning or tingling feeling. They may be swollen and painful. Having a canker sore can make it hard to talk or eat.

Canker sores may hurt for 7 to 10 days. Minor canker sores heal completely in 1 to 3 weeks, but major canker sores can take up to 6 weeks to heal. Some people get another canker sore after the first sore has healed. Most canker sores heal without a scar.

You do not need to see a doctor for most canker sores. They will get better on their own. There are many things you can try at home to relieve the pain caused by your canker sores:

• Eat soft, bland foods that are easy to swallow, such as yogourt or cream soup.
• Cut your food into small pieces or mash or puree it.
• Avoid coffee, chocolate, spicy or salty foods, citrus fruits or juices, nuts, seeds, and tomatoes.
• Drink cold fluids, such as water or iced tea, or eat Popsicles.
• Sometimes fluid touching the canker sore can cause a stinging pain. Use a straw so the fluid doesn’t touch the canker sore.
• Hold ice on the canker sore until it is numb.
• Rinse your mouth with salt water. To make a salt water rinse, dissolve 1 tsp (5 g) of salt in 1 cup (250 mL) of warm water.
• Buy an over-the-counter medicine such as Orabase, Anbesol, or milk of magnesia to put on your canker sores. Use a cotton swab to apply the medicine. Put it on your sores 3 to 4 times a day.
• Take a pain reliever, such as non-steroidal anti-inflammatory drugs (NSAIDs), including ASA (such as Aspirin) or ibuprofen (such as Advil). Do not give ASA to anyone younger than 20 because of the risk of Reye syndrome.

If your canker sores do not feel better after trying these steps at home for 2 weeks, you may need to see your doctor or dentist. He or she may recommend medicines that will help the pain caused by your canker sores. Usually these medicines are swished or gargled in your mouth, or they are painted on the sore. Your doctor may prescribe steroid cream (triamcinolone or fluocinonide) to rub on your canker sore and/or a prescription mouthwash to use.

Talk to your doctor if you have a fever, have trouble swallowing, or if your canker sores keep coming back. You may have another problem that is causing your symptoms.