If you have severe dandruff you should wash your hair at least once everyday using the dandruff shampoo to get the best results.
When I had severe dandruff I washed my hair once everyday to get rid of the majority of dandruff and then after a month of washing the hair everyday using the medicated dandruff shampoo I switched to washing my hair every few days which is enough to keep the hair free of most of the dandruff.
You should also use a dandruff hair conditioner to help with your hair.
I also recommend washing and rinsing your hair with some apple cider vinegar which also helps with dandruff as well.
Another way of getting rid of dandruff is to use some coconut oil and do a hot oil treatment of your hair by heating up the coconut oil to warm and then massaging the coconut oil into your hair.
Leave the coconut oil in your hair overnight and wear a shower cap and then rinse the coconut oil out of your hair the next morning and do the coconut oil hot oil treatment at least twice per week for the best results.
Reducing stress levels and increasing your intake of Probiotics can also help to keep dandruff from getting worse and may even prevent dandruff.