What effects from peroxide and from alcohol mouthwashes?

Black hairy tongue. The name sounds scary but it's harmless. Overuse of mouthrinses can kill the bacteria which normally live on the tongue and allow an overgrowth of yeast. The papillae, rough hair-like projections on your tongue, can overgrow and look like hair. It is treated by discontinuing the mouthrinse, brushing and scrapping the tongue, and antifungal prescription if necessary.