Nonphototoxic

Nonphototoxic is a term used to describe chemical substances that do not cause phototoxic reactions after exposure to light, especially ultraviolet A (UVA) radiation. In pharmacology, toxicology, cosmetics, and plant science, the label is applied to drugs, dyes, essential oil constituents, and other compounds that do not become tissue-damaging photosensitizers when illuminated.

A phototoxic reaction is a non-immunologic tissue injury produced when a substance absorbs light, reaches an excited state, and transfers energy to molecular oxygen or other substrates to form reactive species. Nonphototoxic substances either (a) do not absorb light in the relevant wavelengths, (b) do not generate reactive oxygen species or reactive intermediates, or (c) are rapidly metabolized or sequestered so that damage does not occur.

Assessment of phototoxic potential typically uses in vitro assays such as the 3T3 NRU phototoxicity test, in which cultured cells are exposed to test substance with and without UVA irradiation. OECD guidelines (TG 432) formalize this and related methods as part of preclinical safety evaluation. When phototoxicity is absent, a substance may still carry other types of toxicity that are not dependent on light.

In practice, nonphototoxicity is especially relevant for topical products and plant ingredients. Substances that inherently absorb light and generate reactive species are avoided or processed to remove photosensitizing components. Conversely, photosensitizing constituents such as certain furanocoumarins in some essential oils are avoided to reduce phototoxic risk.