From the manufacturer's website:
Fight dandruff outbreaks with NIZORAL® A-D Anti-Dandruff Shampoo. This powerful shampoo controls flaking, scaling, and itching associated with dandruff. It combines the effectiveness of ketoconazole 1%, a proven dandruff-fighting ingredient, with salon-tested gentleness. Use it just twice a week in between your regular shampoos to control dandruff. It’s gentle enough to use on color-treated, chemically processed, and gray hair, and has a thick, rich lather that rinses clean and leaves your hair smelling fresh.
Simply put, NIZORAL® A-D kills the fungus that causes dandruff. Period. That's because the ingredient called ketoconazole binds to the natural protein of your hair so it keeps working between shampoos with NIZORAL® A-D. "Ketoconazole" keeps nasty dandruff in check so you don't have to keep checking your shoulders.
*These statements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure or prevent any disease.