Ranging from mild and annoying to severe and chronic, canker sores can be a pain. Virtually small ulcers in the mouth, they are thought to be caused by stress or tissue injury. The more complex type of canker sores can be brought on by deeper underlying causes, such as vitamin or mineral deficiencies, compromised immune systems, or gastrointestinal diseases. While simple canker sores typically heal on their own in a few days, larger or longer-lasting sores can be treated with medicated rinses, steroid-based ointment, and prescription medication.