WebMD Answers
Canker sores are not contagious.
Nobody knows what exactly causes canker sores, but many factors may contribute to their development, such as certain medications, trauma to the mouth, poor nutrition, stress, bacteria or viruses, lack of sleep, sudden weight loss, and certain foods such as potatoes, citrus fruits, coffee, chocolate, cheese, and nuts.
Canker sores may also be related to a temporarily reduced immune system because of a cold or flu, hormonal changes, mechanical irritation, or low levels of vitamin B12 or folate. Even biting the inside of the cheek or chewing a sharp piece of food can trigger a canker sore.
Canker sores may result from a genetic predisposition and are considered an autoimmune disease.
About 20% people in the U.S. will have canker sores at some point during their lifetime -- women more often than men.
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