Drinking Listerine, especially in excess, is dangerous because it consists of 22 percent alcohol, according to the Illinois Poison Center Blog. The ethanol alcohol content exceeds that found in beer and wine, and drinking too much of it could lead to drunkenness or alcohol poisoning.
The excess alcohol may also contribute to mouth cancer, and other included chemicals, such as hydrogen peroxide, methanol and eucalyptus oil, can cause problems as well, reports Drug & Alcohol Rehab Asia. Many alcoholics turn to Listerine and other mouthwashes for a drinking fix when they cannot buy or access other beverages. The Illinois Poison Center Blog also notes that small children face significant risks of low blood sugar or coma from drinking significant amounts of Listerine.