Can mouthwashes containing peroxide cause mouth cancer?

Probably not: In japan they showed it could happen, but with 40% peroxide. It's illegal in the usa, here we only get up to 3-5% no proof here.

Doubtful: The most common problem with the chronic use of peroxide is developing a yeast or fungal infection. This is due to the fact that the normal bacterial flora in the mouth has been disrupted and you get an overgrowth.

No proof: No evidence to support. See oral surgeon or ENT doc if you have any of these symptoms: Persistent mouth sore, persistent mouth pain, a lump or thickening in the cheek, a white/red patch on the gums, tongue, tonsil, or lining of the mouth, a sore throat, hoarseness or feeling that something is caught in the throat that does not go away, difficulty swallowing/chewing.