Mouth blisters or sores are small, red cuts or ulcers that appear on the insides of the lips, on gums or under the tongue. The sores, which may have white patches in the middle, are painful and take two to four weeks to heal, states the American Cancer Society.
Mouth sores make the inside of the mouth look red, shiny or swollen, according to the American Cancer Society. These sores may result from dehydration, tobacco or alcohol use, oral infections or lack of proteins. Patients with mouth sores feel a mild burning and dryness in their mouths and experience difficulties eating hot or cold foods.