phototoxicity

Phototoxicity is skin (or eye) damage that occurs when a drug in the body absorbs light, usually from the sun, and turns that light energy into chemical harm. The drug itself is harmless in the dark; it is the combination of drug plus light that does the damage, often appearing as an exaggerated sunburn on exposed skin.

The mechanism hinges on a molecule that can absorb ultraviolet or visible light, called a chromophore, often a flat, conjugated aromatic system. Once excited by light, the molecule can dump its energy onto oxygen to make reactive oxygen species, or react directly, damaging skin cells and tissue. Drugs that reach the skin and absorb light in the UV-A or visible range are the usual suspects.

Discovery teams screen for phototoxic potential early, partly by checking how strongly a candidate absorbs light in the relevant wavelength range, then in cell-based or animal phototoxicity assays. A phototoxic liability does not always kill a program, but it can require strict sun-avoidance warnings on the label and may make a candidate less attractive than a clean alternative.