Canker sores are small sores that appear on the inside of the mouth, especially the cheeks, the inside of the lips, and the tongue. They're also called aphthous ulcers. Rarely, they may develop on the gums or on the roof of the mouth. Small canker sores disappear within 10 days and don't scar. Large ones are less common, take weeks to heal, and can leave scars. Canker sores aren't contagious like cold sores are.
At any given time, 20% of Canadians will have a canker sore. They're most common in women and in people aged 10 to 40 years. Many people get them regularly, at least once a year. In the most severe cases, people get one after another.