Mouth ulcers, sometimes called canker sores, are small, painful sores that can develop inside your mouth - inside your cheeks, on the roof of your mouth, on your tongue, or at the base of your gums. They can be raised, and may be red, yellow, white, or grey in colour.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/what-causes-mouth-ulcers