A mouth ulcer, also known as an aphthous ulcer, 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 yellow center and a red halo. Mouth ulcers can be triggered by factors such as stress, certain foods, or injury to the mouth. Most mouth ulcers heal on their own within one to two weeks. While they are not contagious, they can cause discomfort, especially when eating or speaking. Over-the-counter treatments and home remedies, like saltwater rinses or topical gels, can help alleviate pain and promote healing.