口内炎, also known as canker sores, are small, painful ulcers that develop in the mouth. They can appear on the inside of the cheeks, gums, tongue, or the roof of the mouth. The exact cause is often unclear, but factors like stress, certain foods, and vitamin deficiencies may contribute to their formation. These sores usually heal on their own within one to two weeks. While they are not contagious, they can cause discomfort while eating, drinking, or speaking. Over-the-counter treatments and home remedies, such as saltwater rinses, can help alleviate the pain and promote healing.