The effectiveness of pyrithione zinc depends on the extent of binding to the hair and epidermis, which is a function of time, temperature, concentration and frequency of application. Use twice or three times weekly, with a contact time of two to three minutes.

Pyrithione zinc is included in several ‘medicated’ shampoos, but there is no licensed medicinal product.

Selenium sulphide is used twice a week for two weeks, and then once a week as necessary. Each application should be left on the hair for three minutes.

Cautions and side effects

Pyrithione zinc binds strongly to both the hair and epidermis but does not penetrate into the dermis; long-term use has not been associated with toxicity.

Selenium sulphide appears safe for long-term external use, although it is highly toxic if ingested. Regular use of selenium sulphide shampoo leaves a residual odour of hydrogen sulphide and makes the scalp oily. Hair should not be dyed or permed for at least two days before or after use.

Contact dermatitis and hypersensitivity are possible, but rare with both compounds. Neither should be applied to broken or abraded skin, and contact with eyes should be avoided.

Neither compound is contraindicated in pregnancy or breastfeeding, although the manufacturers of selenium sulphide shampoo advise against its use during the first trimester of pregnancy.

Selenium sulphide preparations are not recommended for children aged under five.