In medicine, the term is principally used for abnormal reactions of the skin, and two types are distinguished, photoallergy and phototoxicity.
Particular medications make the skin more sensitive to sunlight; these include most of the tetracycline antibiotics, heart drugs amiodarone, and sulfonamides.
They are classified as primary if an ingested plant contains a photosensitive substance, like hypericin in St John's wort poisoning and ingestion of biserrula (Biserrula pelecinus)[3] in sheep, or buckwheat plants (green or dried) in horses.
[4] In hepatogenous photosensitization, the photosensitzing substance is phylloerythrin, a normal end-product of chlorophyll metabolism.
[5] It accumulates in the body because of liver damage, reacts with UV light on the skin, and leads to free radical formation.