Pustular psoriasis and the Kobner phenomenon caused by allergic contact dermatitis from zinc pyrithione-containing shampoo.

Zinc pyrithione is a shampoo ingredient that has been shown to be safe and effective for dandruff and scalp psoriasis. It is thought to decrease the cell turnover rate in hyperproliferative dermatoses such as psoriasis, and also has fungistatic and antimicrobial activity, although its exact mode of action is unknown. In psoriasis, external factors, such as trauma, infection and drugs, may provoke aggravated manifestations of psoriatic skin lesions. Rarely, irritant or allergic mechanisms are likely causes of psoriatic flare and Kobnerization. A patient had had stable psoriasis for 25 years and no any other skin disease. Within 20 days, she developed an aggravated scaly erythematous patch on the scalp, where a shampoo had been applied, and simultaneously developed pustular psoriasis on both forearms. Patch testing showed a relevant sensitization to zinc pyrithione, and we observed symptomatic aggravation by provocation testing with zinc pyrithione shampoo. We report a rare case of psoriasis aggravated by the induction of allergic contact dermatitis from zinc pyrithione after using antidandruff shampoo.