Mouth Ulcers commonly known as canker sores are painful lesions in the mouth or at the base of the gums or inside cheeks or lips. In scientific terms, it is the loss of tissue lining in the mouth. These are nothing to worry about as they are non-contagious and go away in around a week or two. However, if these are not healed in the period as said so or are large and extremely painful then it is advised to visit a dentist. The pain sometimes can be to the extent that it can make eating, talking and drinking difficult and uncomfortable.