photostability

Photostability is a product's toughness against light — its ability to sit under lamps or daylight without decomposing. If photolysis is the damage light can do, photostability is the measure of how well a particular formulation, in its particular packaging, resists that damage.

It is a property of the whole product, not just the molecule: the same light-sensitive drug may be photolabile as a clear solution yet acceptably photostable as a coated tablet in an opaque blister. Colour, physical state, opacifiers, UV-absorbing excipients and above all packaging all contribute. Demonstrating photostability therefore tests the product as it will be sold, sometimes both with and without its packaging, to separate the drug's intrinsic sensitivity from the protection the package provides.

The reference protocol is ICH Q1B, which prescribes defined doses of both visible and ultraviolet light and acceptance based on changes in appearance, assay and degradation products. The outcome decides whether special light-protective packaging, storage statements (protect from light) or even in-use precautions during administration are needed.

A practical caveat: photostability findings are condition-specific. A product judged photostable behind amber glass on a shelf may still degrade if the contents are transferred to a clear infusion bag and hung under bright theatre lighting, so light protection must be considered along the whole path to the patient.