A canker sore is a small, painful ulcer that appears inside the mouth, often on the inner cheeks, gums, or tongue. These sores are usually round or oval with a white or yellow center and a red halo. Canker sores are not contagious and can be triggered by factors such as stress, hormonal changes, or certain foods. The exact cause of canker sores is not fully understood, but they may be linked to a weakened immune system or nutritional deficiencies, particularly in vitamin B12, folate, or iron. Most canker sores heal on their own within one to two weeks, but treatments can help alleviate pain and discomfort.