Topical salicylic acid interferes with UVB therapy for psoriasis.

Salicylic acid has been widely used in the topical treatment of psoriasis. Chemically it is closely related to paraaminobenzoic acid. Following in vitro studies indicating that salicylic acid might exhibit relevant UVB absorption, we found that salicylic acid had a clinically pronounced filter effect when applied prior to UVB exposure. The duration of photoprotection after application was more than 12 h, sometimes exceeding 24 h. In a prospective, randomized, double-blind, left-right comparison study in patients with psoriasis between emollients with and without salicylic acid, salicylic acid was shown to decrease the clearing rate significantly.