Canker sores, also called ulcers, are small, shallow injuries that develop on the soft tissues in your mouth or near the base of your gums. Canker sores don't occur on the surface of your lips and they aren't deadly. They can be painful, however, and can make eating and talking difficult. Most canker sores go away on their own in a week or two. Check with the dentist if you have unusually large or painful canker sores that can't be healed.