This paper describes three cases of photoallergy to the phototoxic agent methoxsalen (8-methoxypsoralen). Compared to the usual phototoxic response, less time and less ultraviolet energy were required to evoke this photoallergic reaction. Clinically and histologically, the picture mimicked contact allergy. Closed patch tests, using in vitro ultraviolet irradiated drugs, were positive. This indicates that photoallergy to methoxsalen is a variant of contact allergy. Similar mechanisms appear to explain photoallergy in general.
Fulton JE, Willis I. Photoallergy to Methoxsalen. Arch Dermatol. 1968;98(5):445-450. doi:10.1001/archderm.1968.01610170005001