What is a mouth ulcer? Mouth ulcers or informally known as canker sores are white or red bruises that occur on areas like- inside the mouth, cheek, gums, tongue and even lips. We all have witnessed canker sores at some point in our lives and know how annoying and painful they can get. Generally, mouth ulcers can be self-treated but if it prolongs for, say, three and a half weeks, then it’s a serious condition and you need to get medical supervision from the hospital at that immediate moment.