Canker sores are small sores that appear on the inside of the mouth, especially the cheeks, the inside of the lips, and the tongue. They're also called aphthous ulcers. Rarely, they may develop on the gums or on the roof of the mouth. Small canker sores disappear within 14 days and don't scar. Large ones are less common, take weeks to heal, and can leave scars. Canker sores aren't contagious like cold sores are.
They're most common in women and in people aged 10 to 40 years. Many people get them regularly, at least once a year. In the most severe cases, people get one after another.
Doctors don't know exactly what causes canker sores. They may be hereditary, but doctors have not figured out exactly how people inherit the tendency to get them. Researchers think they may be an overreaction to the Streptococcus bacteria, because the bacteria are often found in the canker sore. People with canker sores often have small injuries from dental injections and toothbrushes in the lining of the mouth. Allergies also may be associated with an increased risk for canker sores.
Canker sores often occur near the time of a menstrual period. They may also be brought on by stress – for example, many students get them during exams. Other possible causes include lack of cyanocobalamin (vitamin B12), folic acid, or iron. Certain foods may be triggers for canker sores or act as nutrients for the Streptococcus bacteria. Rarely, they are associated with celiac disease, Crohn's disease, Behcet's syndrome, or HIV.