Phototoxin

Phototoxins are substances that become toxic or more toxic in the presence of light, especially ultraviolet (UV) radiation. They can cause skin reactions like redness, swelling, and burning sensations, and can be harmful to living organisms by generating reactive oxygen species and other free radicals.

Phototoxins are common in:

• a variety of plants (including food plants where they may be a biological defence):
  • many citruses contain essential oils that are photosensitizers;
  • some herbal remedies (notably St John's wort, though incident rates for this plant are reportedly low);
  • the carrot family of Apiaceae;^[1][2]
• some prescribed medications (such as tetracycline antibiotics); and
• many essential oils, perfumes and cosmetics.

Ingested medications may cause systemic photosensitivity and topically applied medications, cosmetics and essential oils may lead to local (or perhaps systemic) photosensitivity. Para-aminobenzoic acid (PABA), found in some sunscreens, can also cause photosensitivity.

Upon exposure to light, notably light containing ultraviolet radiation, discolouration of the skin (whether as inflammation, lightening or darkening) or rashes may result. In extreme cases, blistering may also occur.