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For severe dandruff, you should take oral medicines apart from topical treatment.
Consider taking tablet Ketoconazole 200 mg once daily for two weeks and apply Candid B lotion (Clotrimazole) on the affected scalp twice daily.
As far as shampoo is concerned, use shampoo containing Ketoconazole every alternate day to start with and then taper slowly as you feel improvement.
Shampoo has to be used for a long time as dandruff and hairfall is a chronic disease. Avoid oiling your scalp.
For further information consult a dermatologist online --> https://www.icliniq.com/ask-a-doctor-online/dermatologist