Is listerine (mouthwash) a carcinogen?

No: No and have seen no literature to suggest that it might be.

No but....: Listerine as well as many other mouthwashes come both with and without alcohol as an ingredient. That tends to dry the mouth and with long term use, and especially when you add smoking to the mix, there is some question as to it's ill effects. For most patients mouthwashes are not an essential part of good oral hygiene. Brushing, flossing and a nutritious diet low in refined sugars are.