What are canker sores? Canker sores — or aphthous ulcers — are small, shallow ulcers that occur in the lining of your mouth. A canker sore starts as a white or yellowish mouth sore with a red border. They’re usually very small (less than 1 millimeter) but may grow to 1/2 inch to 1 inch in diameter., Aphthous ulcers are small, round, shallow lesions that develop on the inside of the mouth or the base of the gums. Aphthous ulcers may also be called aphthous stomatitis or canker sores., Canker sores, also called aphthous ulcers, are small, shallow lesions that develop on the soft tissues in your mouth or at the base of your gums. Unlike cold sores, canker sores don't occur on the surface of your lips and they aren't contagious., Aphthous ulcers are generally round in shape and form in the soft areas of the mouth such as the inside of the lips, the cheeks, or the underside of the tongue. They are benign, non-contagious, and can occur as single ulcers or in clusters., What Is a Canker Sore? A canker sore is a small, shallow, open wound or ulcer in your mouth that can make eating and talking uncomfortable. It is also known as a mouth ulcer or aphthous ulcer., Mouth ulcers are painful sores that can occur anywhere inside the mouth. This leaflet is about the most common type of mouth ulcers, which are aphthous mouth ulcers..