Selsun Shampoo is a medicated shampoo used to treat dandruff and seborrhoeic dermatitis (a greasy, crusting, itching scalp caused by excess oil).
It works against dandruff in two ways:?

By reducing greasiness of the scalp and?
By slowing down growth of the layer of skin cells on the scalp that cause dandruff

Selsun should be used twice a week for the first two weeks and then once a week for the next two weeks. After this first course of treatment, it should not be used more often than is needed to control your scalp condition.
Selsun Shampoo is not recommended for use in children under 5 years of age
If you accidentally swallow Selsun Shampoo, seek medical advice immediately. Swallowing Selsun Shampoo can cause you to feel sick or be sick.