Selsun shampoo is a quick simple and highly effective medical treatment for dandruff selsun shampoo contains 25 wv selenium sulphide which works throughout the day to clear up dandruff and seborrhoeic dermatitis a greasy crusting itching scalp caused by excess oil it works against dandruff in 2 ways by reducing the greasiness of the scalp and by slowing down the growth of the layer of skin cells on the scalp that cause dandruffr directions r not recommended for children under 5