The phototoxicity of methoxsalen (8-methoxypsoralen [8-MOP]) and trioxsalen (trimethylpsoralen) has been compared in humans.
Topically, both drugs evoked equivalently intense reactions at low concentrations after exposure to long ultra-violet rays from the filtered xenon lamp.
Whereas 60 mg of orally administered methoxsalen produced sharp reactions with occasional blistering, 200 mg of trioxsalen had no effect with long ultraviolet rays. Mild phototoxic responses were occasionally obtained with one-hour sunlight exposures after administration of 200 mg, but not less, of trioxsalen.
Since the dose of trioxsalen is usually not more than 20 mg, the claims of therapeutic value in vitiligo probably reflect observer error.
Kligman AM, Goldstein FP. Ineffectiveness of Trioxsalen as an Oral Photosensitizer. Arch Dermatol. 1973;107(3):413-414. doi:10.1001/archderm.1973.01620180067021