Mountain Dew mouth is a term penned by dentists in reference to a form of tooth decay that is caused by drinking excessive amounts of sugary soda pop. The term is widely used in poorer areas of the country, especially the Appalachia Mountains.
While all soda pop, in essence, can cause advanced tooth decay, Mountain Dew has long been directly associated with advanced tooth decay in children and adults, likely due to the higher sugar content (as compared to most other sodas).
Mountain Dew mouth is often compared to a dental condition known as meth mouth, because the rate in which the decay progresses and the affect on the gum issue is very similar.
