Like cold sores, canker sores – also known as aphthous ulcers – can be quite painful and often recur. While cold sores are caused by a virus, the formation of a canker sore may be triggered by multiple factors such as stress, food allergies or a weakened immune system. A canker sore forms in the soft tissues of your mouth and is not contagious, whereas a cold sore usually appears around the lip area outside the mouth and is highly contagious. If a cold sore appears inside the mouth, usually it appears on the non-movable parts such as the roof, rather than the tongue or soft palate.

Reviewed: January 2012