Dandruff affects about 50 million people in the U.S.

Dandruff can have several causes, including:

• Dry skin. Simple dry skin — the kind you get during winter when the air is cold and rooms are overheated — is the most common cause of itchy, flaking dandruff. Flakes from dry skin are generally smaller and less oily than those from other causes of dandruff, and you'll likely have symptoms and signs of dry skin on other parts of the body, such as your legs and arms.
• Irritated, oily skin (seborrheic dermatitis). This condition, one of the most frequent causes of dandruff, is marked by red, greasy skin covered with flaky white or yellow scales. Seborrheic dermatitis may affect not only your scalp, but also other areas rich in oil glands, such as your eyebrows, the sides of your nose and the backs of your ears, your breastbone, your groin area, and sometimes your armpits.
• Not shampooing often enough. If you don't regularly wash your hair, oils and skin cells from your scalp can build up, causing dandruff.
• Psoriasis. This skin disorder causes an accumulation of dead skin cells that form thick, silvery scales. Psoriasis commonly occurs on your knees, elbows and trunk, but it can also affect your scalp.
• Eczema. If you have eczema anywhere on your body, it could also be on your scalp, possibly leading to the development of dandruff.
• Sensitivity to hair care products (contact dermatitis). Sometimes sensitivities to certain ingredients in hair care products or hair dyes, especially paraphenylene diamine (PPD), can cause a red, itchy, scaling scalp. Shampooing too often or using too many styling products also may irritate your scalp, causing dandruff.

Dandruff does not cause baldness.

Dandruff isn't contagious and those white flakes on your shoulders pose no health risk.  Although it's embarrassing -- and the itching can be bothersome -- dandruff isn't harmful.

1 out of 5 people suffer from scalp flaking.

Stop scratching - It’s tempting to scratch an itchy scalp, and doing so may relieve the itch, but it can make things worse. That’s because scratching releases flakes of skin and increases visible dandruff. It can also weaken hair, making it prone to breaking.

Dandruff cannot be cured - but it can be controlled.

To decrease the buildup of dandruff's dead skin cells, try using an antidandruff shampoo daily. Leave shampoo on for 5 minutes, then rinse well. You may need to try several dandruff shampoos to find one that works best for you. If one stops working, try another. If that doesn't help, call your doctor.

If the dandruff flakes you see are greasy and yellow, you may have seborrheic dermatitis. It's an inflammatory skin condition that can occur where there are lots of oil glands, like the scalp and face. Though seborrheic dermatitis is related to hormones, fungus, and even some neurological problems, it's treated the same as dandruff: with antidandruff shampoos. Severe cases may need a prescription steroid or antifungal medicine.

Dandruff Shampoos Defined by Medicine

Dandruff shampoos are classified according to the medication they contain:

• Zinc pyrithione shampoos (such as Selsun Salon, Head & Shoulders). These contain the antibacterial and antifungal agent zinc pyrithione, which can reduce the fungus on your scalp that can cause dandruff and seborrheic dermatitis.
• Tar-based shampoos (such as Neutrogena T/Gel). Coal tar, a byproduct of the coal manufacturing process, helps conditions such as dandruff, seborrheic dermatitis and psoriasis by slowing how quickly skin cells on your scalp die and flake off.
• Shampoos containing salicylic acid (such as Ionil T). These "scalp scrubs" help eliminate scale, but they may leave your scalp dry, leading to more flaking. Using a conditioner after shampooing can help relieve dryness.
• Selenium sulfide shampoos (such as Selsun Blue). These shampoos slow your skin cells from dying and may also reduce malassezia. Because they can discolor blond, gray or chemically colored hair, be sure to use them only as directed, and rinse well after shampooing.
• Ketoconazole shampoos (such as Nizoral). Ketoconazole is a broad-spectrum antifungal agent that may work when other shampoos fail. It's available over-the-counter as well as by prescription.