Cold sores are small, painful blisters around the outside of the mouth, nose and sometimes the chin. Sometimes they appear on the gums or roof of the mouth. They are red, raised and often tingle a day or two before the actual blister erupts. They often last for a week or two when left untreated. Once the blisters break, they ooze and scab over with a yellowish crust. Blisters located on the inside of the cheeks or under the tongue are not cold sores. They are canker sores.