Dandruff (Pityriasis simplex capillitii):
Affects approximately 50% of the population in some mild to severe form.

My dermatologist told me the best way to get rid of dandruff is to:
1. Alternate between three different shampoos with different main active ingredients.
2. Shampoo in the sink and leave it on for 5 min. before taking a shower and rinsing it out.

Dandruff is usually caused by the body's natural reaction to Malassezia globosa (M. globosa), the dandruff-causing fungus that lives on the scalp of 100 percent of the adult population.
Dandruff can also be a manifestation of an allergic reaction to chemicals in hair gels, sprays, and shampoos, hair oils, or sometimes even dandruff medications like ketoconazole.

Fungicides such as ketoconazole, zinc pyrithione and selenium sulfide reduce loose dander by killing the fungus.

Fungus' include: Malassezia globosa, Malassezia furfur and Pityrosporum ovale yeast.

Active Ingredients
The following active ingredients and shampoos are effective against fungus.

• Selenium sulfide - Anti-fungal and anti-seborrheic - regulates keratinization - Selsun Blue and Head & Shoulders Intensive Treatment Dandruff Shampoo
• Zinc pyrithione (ZPT) - Anti-fungal and Antibacterial - Extra Strength Dandruff Shampoo | Head & Shoulders for Men
• Ketoconazole - An imidazole topical anti-fungal. Effective against seborrheic dermatitis as well. Nizoral A-D

Salicylic acid, and coal tar belong to a class of drugs known as keratoplastics. They work by causing the skin to shed dead cells (peeling agent) from its top layer and slow down the growth of skin cells.

Salicylic acid - Extra Strength Protection | Denorex Shampoo - T/Gel therapeutic conditoner
Coal Tar - T/Gel Original Formula, Denorex Therapeutic Protection

Others:
Valisone (Betamethasone Valerate) is commonly prescribed for dandruff.

Side Effects:
Some of the ingredients of dandruff shampoos can cause skin irritation and other side effects.
Coal tar is thought to be a carcinogen at high doses, although this isn't thought to be a problem at the low doses found in anti-dandruff shampoos.
See The Side Effects Of Anti-dandruff Shampoos at LiveStrong.com
and Dandruff Drugs - adverse event reports at DrugInformer.com

Terms:
- Dandruff - Flakes of dry skin on the scalp. Affects approximately 50% of the population in some mild to severe form. Usually caused by a malassezia fungus.
- Psoriasis is a noncontagious skin condition that produces red, dry plaques of thickened skin. The dry flakes and skin scales are thought to result from the rapid proliferation of skin cells that is triggered by abnormal lymphocytes from the blood.
- Seborrheic dermatitis affects a much smaller percent of the population and is characterized by severe flaking, inflammation, redness and itchiness. Caused by a fungus like dandruff.
- Imidazole topical antifungals such as ketoconazole act by blocking the biosynthesis of ergosterol, the primary sterol derivative of the fungal cell membrane.
- Keratin - a family of scleroproteins that are the main constituents of epidermis, hair, nails, and horny tissues.
- Keratinization - a process by which epithelial cells lose their moisture and are replaced horny tissue.

Links:
Dandruff - Wikipedia Dandruff, scalp psoriasis, seborrheic dermatitis, ketoconazole, psoriasis treatment, psoraisis, eczema, eczema treatment
Dandruff Drugs
Choosing a Dandruff Shampoo
Neutrogena - T/Gel Dandruff Shampoo & Scalp Treatments - Selection guide
Dandruff Shampoo | LIVESTRONG.COM
The Side Effects Of Anti-dandruff Shampoos at LiveStrong.com
bestpractice.bmj.com/best-practice/pdf/patient-summaries/patient-summary-1203678764268.pdf
Ketoconazole Dandruff Shampoo Vs. Head and Shoulders: Which is Best? - Yahoo! Voices