What causes canker sores?

According to WebMD, as of 2014, the cause of most canker sores is unknown. However, their appearance is linked to several factors, including eating acidic fruits and vegetables as well as damage to the lining of the mouth.

WebMD states that eating several kinds of fruits and vegetables, including lemons, apples, pineapples, tomatoes, figs, oranges and strawberries, can trigger canker sores or make existing ones worse. An injury to the mouth, especially from constant irritation caused by braces or other dental appliances, is thought to cause canker sores. Also, canker sores are sometimes linked to an underlying health condition, such as celiac disease, deficiency of vitamin B-12 or iron, or an impaired immune system.