People who have a mouth ulcer or have had one in the past are being issued a stark health warning. Mouth ulcers , sometimes referred to as canker sores or aphthous ulcers, are common and most are harmless. They can be painful lesions or sores that develop on the soft tissues of the mouth, including the roof of the mouth, inner cheeks, lips, tongue, and gums. Usually round or oval in shape, they have a red, inflamed border and a white, yellow, or grey centre. They are usually harmless, non-