Well, use it to rinse your scalp and hair, that is.
Mouthwash works on dandruff primarily because of the hydrogen peroxide in them. Hydrogen peroxide has antiseptic properties which helps kill of fungi that cause dandruff and hair fall.
How to do it: Wash your hair with regular shampoo. Rinse with cold water and work about a capful of the mouthwash into your scalp.
Finally, rinse it off again with cold water and you’re done. (Alternatively, you can also add your conditioner, then rinse.)