Phototoxin

Phototoxins are toxins that can cause allergic reactions in particularly susceptible individuals and which can cause dangerous photosensitivity in a much broader range of subjects.

Phototoxins are common in: 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.
 * 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;
 * some prescribed medications (such as tetracycline antibiotics); and
 * many essential oils, perfumes and cosmetics.

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.

Uses
The marigold plant produces the phototoxin alpha-terthienyl, which functions as a nematicide. When exposed to near ultraviolet light, such as in sunlight, alpha-terthienyl generates the toxic singlet oxygen. Alpha-terthienyl results in damage to the respiratory, digestive and nervous system of larvae, resulting in 100% death rates in concentrations of 33 ppb. This makes it an interesting natural insecticide.

Rose bengal and other singlet oxygen generating phototoxins are also used in synthetic organic chemistry. They have also found use in photodynamic therapy, where the toxin is activated by intense light to destroy cancer cells.