Selenium disulfide

Selenium disulfide, also known as selenium sulfide, is a chemical compound and medication used to treat pityriasis versicolor, seborrhoeic dermatitis, and dandruff. It is applied to the affected area as a lotion or shampoo. Dandruff frequently returns if treatment is stopped.
Side effects include hair loss, irritation of the skin, weakness, and feeling tired. Use is not recommended in children less than 2–5 years old. Use in pregnancy or breastfeeding has not been studied. Selenium disulfide is an inorganic compound with the chemical formula SeS2.
Selenium disulfide was approved for medical use in the United States at least as early as 1951. It is on the World Health Organization's List of Essential Medicines. Selenium disulfide is available as a generic medication and over the counter.
Medical uses.
Selenium disulfide is sold as an antifungal agent in shampoos for the treatment of dandruff and seborrheic dermatitis associated in the scalp with fungi of genus "Malassezia". In the United States, a 1% strength is available over-the-counter, and a 2.5% strength is also available with a prescription. In Canada, the 2.5% strength is available over-the-counter. At the 2.5% strength, selenium disulfide is also used on the body to treat Tinea versicolor, a type of fungal skin infection caused by a different species of "Malassezia". It has been suggested to be effective as a treatment for hyperkeratosis.
Side effects.
Selenium disulfide can cause discoloration of the hair and alter the color of hair dyes. It may also discolor metallic jewellery.
Chemical composition.
Selenium disulfide has a composition that approximates to SeS2 and is sometimes called selenium sulfide. However, as used in proprietary formulations, it is not a pure chemical compound but a mixture where the overall Se:S ratio is 1:2. The compounds are Se–S rings containing a variable number of S and Se atoms, SenS8−n.
Many selenium sulfides are known as indicated by 77Se-NMR spectroscopy.
History.
Selenium monosulfide, along with elemental selenium and sulfur, has been used in medicinal preparations in the past, causing confusion and contradiction as to exactly what form selenium is in any given topical preparation.
Society and culture.
In the film Evolution selenium was mentioned as an active ingredient of Head & Shoulders. A group of academics, therefore, tried to use this brand of shampoo to stop an alien invasion after discovering that the alien life form was sensitive to selenium.