Canker sores, also known as mouth ulcers, are painful lesions that develop in the mouth, at the base of the gums, or within the cheeks or lips. It is the loss of tissue lining in the mouth in scientific terms. They aren’t infectious and will go away in a week or two, so don’t be worried. However, if they do not heal within the time frame given, or if they are broad and serious, you should seek medical attention. Pain can also be so severe that feeding, talking, and drinking becomes difficult and unpleasant.