Canker sores are small shallow ulcers in the mouth that can be caused due to issues, such as stress, tissue injury, a virus, irritation to spicy foods, or biting the inside of the mouth. There are foods, such as citrus and acidic fruits and vegetables, that can cause canker sores. To prevent canker sores it is recommended to brush your teeth regularly to keep the mouth free from irritants, avoid foods that cause irritation in the mouth, especially acidic and spicy foods, and avoid irritation from chewing gum. Hard and crunchy food can also cause canker sores.
The sores usually go away without any treatment but there are over the counter products that can provide temporary relief and healing. If the sores are unusually large, are spreading, last longer than three weeks, won't go away, cause high fevers, or the pain doesn't allow you to drink enough fluids, then it is recommended to visit a doctor as soon as possible. A doctor will be able to prescribe a medication that can help.