Thanks for the query.
Severe dandruff could indicate dandruff itself which is significant or can indicate beginning of some other skin condition like Seborrheic dermatitis or Scalp psoriasis. An antifungal shampoo should work.
Your dermatologist will decide a treatment plan for you after a consultation.
using an antifungal shampoo everyday for first week and then reduce the application to twice a week for maintenance. You should keep the shampoo at-least for 5 minutes on the scalp for it to act. This should solve your problem. However if problem persists, consult a Dermatologist for proper diagnosis and treatment and to rule out any other skin condition.