Canker sores develop inside the mouth as small white or gray sores that have a red border. They are not contagious. They may occur as one sore or several. In some cases, the cause is unknown, but trauma to oral soft tissues is a common cause of canker sores. Canker sores usually heal on their own after one or two weeks. They are painful; over-the-counter topical anesthetics and antimicrobial mouth rinses may provide temporary relief. Spicy, salty or acidic foods, such as citrus fruits or juices, can irritate the sore.