Phototoxicity, also called photoirritation, is a chemically induced skin irritation, requiring light, that does not involve the immune system.

This state can undergo intersystem crossing with neighboring molecules in tissue, converting them to toxic free radicals.

Cytotoxicity is expressed as a concentration-dependent reduction of the uptake of the vital dye - Neutral Red.

The reliability and relevance of the 3T3 NRU PT have been evaluated, and the test has been shown to be predictive when compared with acute phototoxicity effects in vivo in animals and humans."

Taken with permission from [1] Several health authorities have issued related guidance documents, which need to be considered for drug development: When performing microscopy on live samples, one needs to be aware that too high light dose can damage or kill the specimens and lead to experimental artefacts.