A canker sore is a small, painful sore that develops inside the mouth. Also known as aphthous ulcers, these shallow sores can be found on or under the tongue, inside your cheek or lip, at the base of the gums or on the soft palate. Canker sores can appear on their own or in groups and are recurrent. Although canker sores can be painful, they will usually clear on their own within one to two weeks. Although often confused, canker sores are not like cold sores, which are caused by the herpes virus. Unlike a cold sore, canker sores do not appear on the lips and are not contagious. What causes canker sores? Although the exact cause of this type of mouth ulcer is unknown, hereditary factors, stress or tissue injury are the likely causes …