Irradiation with long ultraviolet rays two hours after ingestion of 90 mg of trioxsalen produced pigmentation without a preceding visible inflammatory reaction. Histologic examination demonstrated an increase in melanocytes in the absence of epidermal damage. Melanogenesis was thus stimulated independently of a phototoxic reaction. The term "drug photopigmentation" has been proposed for this phenomenon.
Kaidbey KH, Kligman AM. Photopigmentation With Trioxsalen. Arch Dermatol. 1974;109(5):674-677. doi:10.1001/archderm.1974.01630050020004