What causes dandruff?

Dandruff is understood to be the result of a combination of factors. As skin grows, skin cells are pushed outward where they eventually die and flake off the head. In most people, these flakes of skin are too small to be visible. However, certain conditions cause cell turnover to be unusually rapid, especially common in the scalp. In people with dandruff, skin cells may die and be replaced about once every two weeks, as opposed to around once a month in people without dandruff. The result is that dead skin cells are shed in large clumps, which appear as small, white or grayish patches on the scalp.

The most common cause of dandruff is the fungus Malassezia furfur. This fungus is found naturally on the skin surface of both healthy people and those with dandruff. The fungus causes irritation in areas where sebum is found, areas with many sebaceous glands (on the scalp, face and upper part of the body). When Malassezia furfur grows too rapidly, the natural renewal of cells is disturbed and dandruff appears with itching. Other fungi may have a similar role in causing dandruff, as may certain bacteria.

Dandruff is not in fact caused by a dry scalp.

Mild dandruff may be caused by overactive sebaceous glands. Other causative factors include:

family history
food allergies
excessive perspiration
use of alkaline soaps
yeast infections
stress
Even the season of the year can contribute to the problem: Cold, dry winters are notorious for bringing on dandruff or making it worse.

Symptoms of dandruff can also be aggravated by:

exposure to dust
UV light
harsh shampoos
hair dyes
In rare cases, dandruff may be caused by over use of hair gel or spray.

The current drug of choice is Nizoral (Ketoconazole), though coal tar and other less expensive shampoo choices may be made as a first attempt for milder forms. NB Coal tar has been in increasing disfavor in the United States due to suspicions that it causes cancer.