An aphthous ulcer, also known as a canker sore, is a small, painful sore that develops inside the mouth. These ulcers can appear on the inner cheeks, gums, tongue, or the roof of the mouth. They are usually round or oval with a white or gray center and a red halo. The exact cause is unknown, but factors like stress, certain foods, and nutritional deficiencies may contribute to their development. Aphthous ulcers are not contagious and typically heal on their own within one to two weeks. Treatment focuses on relieving pain and discomfort, often using topical medications or mouth rinses. Maintaining good oral hygiene and avoiding irritants can help prevent future occurrences.