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Psoralens have been used for decades to stimulate repigmentation in cases of vitiligo. Recently, there has been considerable interest in the treatment of psoriasis with orally administered psoralens and subsequent exposure to longwave ultraviolet light (UVA, 320 to 400 nm). The relationship between the responses of these skin diseases to psoralen therapy and the striking erythemogenic and melanogenic effects of photoactive psoralens on normal skin is unknown. No relationship has yet been established between any of these cutaneous effects and the known formation of photoadducts of thymine and psoralen, nor between cutaneous effects of psoralen therapy and the transiently decreased DNA synthesis that occurs when psoralentreated cells are exposed to UVA.
Double-blind acute toxicity studies performed on human volunteers and studies of large numbers of vitiligo cases have disproved the early concerns about the possible hepatotoxicity of psoralens. Repeated exposure to high doses of UVA is carcinogenic to certain laboratory
Parrish JA. Photochemotherapy of Psoriasis. Arch Dermatol. 1976;112(1):35-36. doi:10.1001/archderm.1976.01630250007001