30th July 2026

Background

Photosensitivity also known as photodermatoses relate to abnormal skin reactions secondary to sun exposure. Photodermatoses can be split into the following major categories: idiopathic, endogenous/exogenous, photoexacerbated dermatoses and photosensitive genodermatoses. 

Idiopathic photodermatoses refer to photodermatoses where the underlying pathology is unknown. The most significant endogenous dermatoses is porphyria where there is an abnormality in one of the eight enzymes involved in haem biosynthesis. Exogenous photosensitivity refers to an abnormal sun reaction secondary to exposure to drugs or chemicals. Photoexacerbated dermatoses are skin disorders that are worsened by sun exposure such as dermatomyositis or systemic lupus erythematosus and genodermatoses, are genetic conditions that also trigger a photosensitive reaction. 

This chapter will focus on exogenous photosensitivity. They can be split into phototoxic or photoallergic reactions. The majority of drug reactions are phototoxic.