Methoxsalen (or Xanthotoxin, 8-methoxypsoralen) sold under the brand name Oxsoralen among others, is a medication used to treat psoriasis, eczema, vitiligo, and some cutaneous lymphomas in conjunction with exposing the skin to ultraviolet (UVA) light from lamps or sunlight.

Methoxsalen modifies the way skin cells receive the UVA radiation, allegedly clearing up the disease.

The scale was developed after patients demonstrated symptoms of phototoxicity after oral ingestion of methoxsalen followed by PUVA therapy.

The biosynthetic route then continues with the activation of dimethylallyl pyrophosphate (DMAPP), produced via the mevalonate pathway, to form a carbo-cation via the cleavage of the diphosphates.

[11] Author John Howard Griffin (1920–1980) used the chemical to darken his skin in order to investigate racial segregation in the American South.