Methoxsalen is a naturally occurring photoactive chemical found in the seeds of the Ammi majus (Umbelliferae) plant and in the roots of Heracleum candicans. It belongs to a group of compounds known as psoralens or furocoumarins.Methoxsalen is a naturally occurring photoactive chemical found in the seeds of the Ammi majus (Umbelliferae) plant and in the roots of Heracleum candicans. It belongs to a group of compounds known as psoralens or furocoumarins.