What Causes Dandruff? P. ovale is a fungus found on every human head. Climate, heredity, diet, hormones and stress impact how your body reacts to it. For dandruff sufferers, the natural process of scalp-cell renewal is accelerated when fighting off P. ovale, causing dead cells to fall off faster. This is a leading cause of dandruff and all the symptoms that go with it - the flaking, scaling and itching. How Does NIZORAL A-D Work? 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 dandruff in check so you don't have to keep checking your shoulders.