Dandruff is believed to be caused by overgrowth of yeasts such as Pityrosporum ovale which live on normal healthy human skin.
Also known as pityriasis simplex capillitii, dandruff is sometimes caused by frequent exposure to extreme heat and cold.
The scalp specific fungus, Malassezia globosa, causes local irritation resulting in hyperproliferation of the cells, or keratinocytes, which form the outer layer of the skin.
These form scales which accumulate and are shed as dandruff flakes.
Anti dandruff shampoos work by three mechanisms. Ingredients such as coal tar are antikeratostatic and they inhibit keratinocyte cell division.
Detergents in the shampoo are keratolytic, they break up accumulation of scale.
Finally, antifungal agents such as ketoconazole inhibit growth of the yeast itself.
Other components in anti dandruff shampoo such as selenium sulphide also inhibit yeast growth and therefore scaling.