A naturally occurring substance isolated from the seeds of the plant Ammi majus with photoactivating properties. As a member of the family of compounds known as psoralens or furocoumarins, methoxsalen's exact mechanism of action is unknown; upon photoactivation, methoxsalen has been observed to bind covalently to and crosslink DNA. or A drug used together with UV light to treat psoriasis, vitiligo, and skin nodules of cutaneous T-cell lymphoma. It is also being studied in the treatment of graft-versus-host disease. It belongs to the family of drugs called psoralens and furocoumarins.