Mouth ulcers — also known as canker sores — are normally small, painful lesions that develop in your mouth or at the base of your gums. They can make eating, drinking, and talking uncomfortable. Women, adolescents, and people with a family history of mouth ulcers are at higher risk for developing mouth ulcers. Mouth ulcers aren’t contagious and usually go away within one to two weeks. However, if you get a canker sore that is large or extremely painful, or if it lasts for a long time without healing, you should seek the advice of a doctor.