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Canker sores are small, white sores inside your mouth where the top layer of the skin is damaged. They are very sore but usually don't last long.

If you keep getting bouts of canker sores every few months, weeks or even every few days, doctors say you have recurrent canker sores.¹ You may also hear them called aphthous ulcers.

Doctors divide recurrent canker sores into three main types, depending on how big they are and where in your mouth you get them.

Most people get minor canker sores. These are round or oval, and usually less than 5 millimeters (a little less than 1/5 of an inch) across. They tend to be grayish-white, with redness around them, and are usually on the inside of the lips or cheeks, or on the floor of the mouth. People usually get one to five sore at a time.²

Major canker sores are less common than minor canker sores, and they are oval and larger. They may be 1 to 3 centimeters (nearly 1/2 to 1 1/4 inches) across. They often happen on the lips or toward the back of the roof of the mouth (the soft palate), but they can be anywhere in the mouth. People usually get 1 to 10 sores at a time.¹

Some people get lots of small, painful sores called herpetiform sores. These sores can occur anywhere in the mouth. You may have as many as 100 at a time, each measuring 2 to 3 millimeters (about 1/8 of an inch) across. Some join together to form large, irregularly shaped sores.¹

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.