How do doctors tell if a canker sore is infected?

Pain and swelling: Canker sores rarely get infected. If you have swelling, a lot of pain, or it deos not heal in about a week to 10 days it is time to see a dentist and find out what is going on in your mouth.

Canker sores: Canker sores are the most common oral lesions. Individual ulcers typically last 1-2 weeks only. They can be painful and can make eating and talking very difficult, but not infected.