The exact cause of canker sores is not known but according to some experts immune system problems, bacteria or viruses may be involved. The risk of developing these ulcers may be inherited. Other factors that possibly increase the risk of canker sores are stress, trauma, allergies, cigarette smoking, iron or other vitamin deficiencies. Some systemic diseases which may be associated with these ulcers are connective tissue diseases such as lupus or Behçet's syndrome, which affect the eyes and genitals as well as the mouth.
You may have one or many ulcers at the same time and they are often recurrent. If you have multiple ulcers they are usually scattered across the lining of the mouth, and are not seen in clusters. In most people the lesions are one to three in number at each episode, but some people may get several ulcers at a time. Canker sores should not be confused with cold sores. Cold sores are caused by herpes virus and are contagious. Canker sores are not transmitted from one person to other as the cold sores. Canker sores are seen inside the mouth, whereas cold sores generally crop up outside the mouth.
Recurrent aphthous stomatitis occurs in about 17% of the people.
These ulcers are classified into three categories:
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