It’s easy to confuse cold sores and canker sores.
Unlike a cold sore, a canker sore is not contagious, and it appears on the inside tissues of the mouth rather than the outer surface of the lip (which is where cold sores appear). A canker sore, also known as an apthous ulcer, looks like a small, round or oval lesion that has a white or yellowish center surrounded by red. They tend to be indented, like a crater, rather than raised, like a bump.
Canker sores most often appear on the inside of the cheeks and lips or at the base of the gum. Canker sores are not usually associated with bleeding gums, so if you are experiencing bleeding gums you should see your dentist to be evaluated for possible gum disease.
The majority of canker sores are mild. Mild canker sores are less than one-third of an inch long and usually heal on their own after a few weeks. But major apthous ulcers, defined as larger than 10 mm in size, can take more than a month to heal and can cause scarring when they finally do heal.
So it’s important to visit your doctor or dentist if you have a canker sore that has persisted for more than a few weeks. These large ulcers are most common in young adults after puberty, and they are more likely to recur than smaller sores. Older adults are more prone to herpetiform lesions, in which dozens of tiny lesions group together to form a large ulcer.
The exact cause of canker sores remains uncertain, but possible factors include an allergic reaction to bacteria in the mouth, a minor injury to the inside of the mouth due to dental work or poorly fitting dental appliances, food allergies or health problems, such as celiac disease and inflammatory bowel diseases.
Although most canker sores resolve on their own, if you have a large or stubborn lesion, your dentist may prescribe an antibiotic mouth rinse, topical paste to apply to the lesion, or a nutritional supplement if poor nutrition may be the cause of the canker sore.