?Canker sores: Canker sores are not contagious. They may look like a pale or yellow ulcer with a red outer ring. You may have one, or a group of them. Women seem to get them more than men. The cause of canker sores is not clear. It may be due to: a weakness in your immune system for example, from the cold or flu , stress or lack of vitamins and minerals in the diet, including vitamin B12 or folate (folic acid).
Answered 6/24/2017
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