Seborreic dermatitis, also known as dandruff, causes your scalp to shed dry white flakes of dead skin. Not only is dandruff visible and embarrassing, often leaving a cascade of flakes on dark clothing, it can be painfully itchy. A number of dandruff treatment shampoos are available over the counter, including some that were formerly prescription-only.

The most common medications in commercial dandruff shampoos are zinc pyrithione, an antifungal found in Selsun Blue, Head & Shoulders and Wal-Mart's Equate brand; selenium sulfide, found in Head & Shoulders Intensive, Walgreens Menthol Medicated and Rugby; salicylic acid, found in Neutrogena T/Sal and Scalpicin; ketoconazole, found in Nizoral and Nizoral A-D; and coal tar, found in Ionil-T and usually used for dandruff caused by psoriasis.

Natural herbal remedies are an alternative to chemical treatment of dandruff, and several environmentally-friendly cosmetics manufacturers have developed effective dandruff shampoos. Jason Cosmetics' Dandruff Relief Shampoo contains rosemary, tea tree oil, neem and colloidal sulfur. Lush's Soak and Float solid shampoo uses cade, an antimicrobial oil with a distinctive campfire aroma. Those who dislike dandruff shampoos for their medicinal smell may wish to try a natural remedy first. Tea tree oil and rosemary are smells many people find pleasant, and although cade may be somewhat unpleasant, its scent fades as your hair dries.

According to the Mayo Clinic, zinc pyrithione is effective for seborrheic dermatitis because it reduces the fungus present on your scalp. Coal tar, a byproduct of coal manufacturing, slows the rate at which scalp skin cells die and shed. Salicylic acid exfoliates the scalp, while selenium sulfide slows the rate of skin cell death.
Antifungal ketoconazole is the one to try when all other remedies have failed. Formerly available only by prescription, you can now get ketoconazole shampoo over the counter.

MayoClinic.com states that the causes of dandruff include shampooing too often or not often enough, dermatitis due to shampoos, conditioners and styling products, and skin disorders such as eczema and psoriasis. Another culprit is Malassezia furfur, a natural fungus found on all human scalps. Malassezia feeds on sebum, so in those whose scalps produce excess oil, Malassezia can grow out of control. Keeping your scalp clean is essential in dandruff caused by Malassezia overgrowth.

Dandruff medications, particularly ketoconazole, can be irritating to the scalp. Drugs.com states that ketoconazole's potential side effects are numerous and include severe allergic reactions, nausea, depression and irregular heartbeat. Salicylic acid can be very drying and should always be used with a dandruff conditioner. Zinc pyrithione can cause an allergic reaction including hives and itching. Always read about the potential side effects of any medication you are using and discontinue use if you experience them.