Pityrosporum ovale, and dandruff

Pityrosporum ovale
Inflammation
Some shampoos slow down the production of skin cells; others remove the cell overgrowth and keep the dead cells from clumping together and/or reduce the inflammation on the scalp.

These OTC products include salicylic acid-based shampoos (such as X-Seb T and Sebulex), as well as those containing selenium sulfide (Selsun Blue), pyrithione zinc (Head & Shoulders), tar (Neutrogena T/Gel), and the antifungal ketoconazole (Nizoral A-D).
"Some people find that using two different types of shampoo -- for example, alternating a tar shampoo with a non-tar shampoo -- works best," says Beutner.

For particularly stubborn cases, your doctor may recommend a more potent dandruff shampoo, such as a prescription-strength ketoconazole product, or a prescription shampoo containing cortisone-type medicine.
Shampoo frequently. No matter what dandruff shampoo you choose, use it as often as once a day, says Shupack. Rub it into the hair thoroughly, leave the lather on for several minutes before rinsing, relather, and rinse again.
Let the sun shine. Exposure to ultraviolet light could have an anti-inflammatory effect when dandruff is present, which may explain why dandruff is less common in summer. Even so, the sun's benefits may be limited since the hair acts as a natural sunscreen.
Dandruff is thought to be caused by a form of a skin condition called eczema, which causes increased shedding of normal scalp skin cells.

Your scalp can be either excessively dry or oily.
If an antidandruff shampoo helps stop dandruff for a while but the dandruff problem returns, rotate shampoos.

Rub your scalp. When you wash your hair, lather once, rinse, then lather a second time and rub your scalp vigorously as you shampoo. This will help dislodge excess cells. Be careful not to rub so energetically that you damage your scalp.
Let it soak. After you lather the second time, let the medicated shampoo sit on your head for 5 minutes. This will give the antidandruff shampoo time to work.
seborrheic dermatitis. It's a form of eczema that mostly affects people with oily or waxy skin
The shampoos should be used at least twice a week but can be used daily if needed.
It's not just the brand on the bottle that makes a dandruff shampoo. Many contain tar, salicylic acid, selenium sulfide, sulfur, and pyrithione zinc -- ingredients that give the shampoos their punch. They don't work forever, though. Once you notice that your shampoo is no longer doing the trick, switch to another type. When that one ceases to offer results, switch to another, or go back to the first one.
Shampoos containing the yeast-fighter ketaconazole could do the job, since yeast may be the cause of your dandruff.
Then there are prescription cortisone creams and lotions. Over-the-counter hydrocortisone creams or ointments could also work. But be sure to work with your doctor to circumvent damage to skin and stretch marks, or a rebound effect, in which the dandruff returns as soon as you stop using the cortisone.
The FDA has approved only five active ingredients as safe and effective for treating dandruff: coal-tar preparations, pyrithione zinc, salicylic acid, selenium sulfide, and sulfur (the FDA also recognizes a combination of salicylic acid and sulfur as an effective preparation). All other substances have been banned as active ingredients in dandruff shampoos, so read the label.

Most doctors recommend that dandruff sufferers use a medicated shampoo daily, leaving the lather on for at least 10 minutes. Be sure to rinse thoroughly; shampoo and soap residue can actually aggravate skin problems. After your dandruff has cleared up, use the medicated shampoo no more than once or twice a week, because it is too harsh for daily use. When possible, let your hair dry naturally instead of blowing it dry.
Brushing your hair with a natural-bristle brush can also prove beneficial. Brush your hair from your scalp outward with steady, firm strokes. This will carry oil from your scalp, where it can cause dandruff, along the hair strands, which need the oil to stay shiny and healthy.
Sometimes dandruff is confused with dry skin (flaking caused by peeling sunburn on the scalp), or psoriasis, a chronic skin disease that causes fairly prominent scaling.

I'd recommend using a dandruff shampoo containing the active ingredient pyrithione zinc (such as Head and Shoulders, Denorex, or Pert Plus Dandruff Shampoo). Other such shampoos (for example, Selsun Blue) contain the active ingredient selenium sulfide, which is equally effective but can be more irritating to the skin.
Don't stop using the shampoo when the dandruff clears up. You'll need to continue using it twice a week to suppress the flaking.