The antifungal agent, voriconazole, is associated with phototoxicity and photocarcinogenicity. Prior work has indicated that voriconazole and its hepatic N-oxide metabolite do not sensitize keratinocytes to ultraviolet B (UVB). Clinical observations have suggested that ultraviolet A (UVA) may be involved.To determine the photochemistry and photobiology of voriconazole and its major hepatic metabolite, voriconazole N-oxide.Voriconazole and voriconazole N-oxide were spectrophotometrically monitored following various doses of UVB. Cultured human keratinocytes were treated with parental drugs or with their UVB photoproducts, and survival following UVA irradiation was measured by thiazolyl blue metabolism. Reactive oxygen species (ROS) and 8-oxoguanine were monitored by fluorescence microscopy.Voriconazole and voriconazole N-oxide have varying UVB absorption but do not acutely sensitize cultured human keratinocytes following UVB exposure. However, sustained UVB exposures produced notable dose- and solvent-dependent changes in the absorption spectra of voriconazole N-oxide, which in aqueous solution acquires a prominent UVA absorption band, suggesting formation of a discrete photoproduct. Neither the parental drugs nor their photoproducts sensitized cells to UVB although all but voriconazole N-oxide were moderately toxic to cells in the dark. Notably, both voriconazole N-oxide and its UVB photoproduct, but not voriconazole or its photoproduct, additionally sensitized cells to UVA by greater than three-fold relative to controls in association with UVA-induced ROS and 8-oxoguanine levels.Voriconazole N-oxide and its UVB-photoproduct act as UVA-sensitizers that generate ROS and that produce oxidative DNA damage. These results suggest a mechanism for the phototoxicity and photocarcinogenicity observed with voriconazole treatment.