Photosensitivity due to drugs.

Photosensitivity reactions induced by drugs may be phototoxic or photoallergic in nature. Acute phototoxic reactions are by far the more common, and are generally characterised by erythema and oedema followed by hyperpigmentation and desquamation. Chronic repeated injury of this type may result in fragility, blistering and milia formation or even actinic keratoses and skin cancers. The photochemical mechanisms involved differ with the chemical photosensitiser involved. They include photoaddition of the chemical to biological targets such as DNA, the formation of toxic products due to absorption of the action spectrum by the photosensitising molecule, or the activation of toxic oxygen species or free radicals. Subsequent activation of the complement pathways may participate in the photoresponse to certain agents. Photoallergic reactions are uncommon. They represent an acquired altered reactivity dependent on a circulating antibody or a cell-mediated hypersensitivity process. Clinically, they are characterised by an immediate wheal and flare or a delayed papular to eczematous process. Some of the same drugs which cause phototoxic responses occasionally produce photoallergic reactions.