A canker sore or aphthous ulcer is a type of oral ulcer caused by a break in the mucous membrane, resulting in a painful open sore found inside the mouth or upper throat. For cases of recurring or multiple ulcers, the condition is also known as aphthous stomatitis. This type of ulcer more commonly affects young adults, and those who have family members who have experienced this type of ulcer have a greater tendency of developing them. Physical or chemical trauma, infections, and food sensitivity are also possible causes.