A canker sore – also known as an aphthous ulcer – is a type of mouth sore that forms on the soft tissues in your mouth, such as on or under your tongue and inside your cheeks or lips.1Canker sores are also commonly referred to as mouth ulcers.2 There’s no difference between a canker sore and a mouth ulcer besides their name.2 Cankers are typically shallow, round or oval ulcers that vary in size. They may appear whitish or yellowish with a red border.2 Canker sores tend to be painful, making it difficult to eat and talk, and large ones can take a while to heal.3 Mouth ulcers are typically caused by trauma, such as aggressive tooth brushing, eating sharp-edged foods such as potato chips, or orthodontic braces and wires that rub against the inner cheek. Other triggers for canker sores include stress, diseases of the immune system and vitamin deficiencies. Mouth ulcers are not contagious.