photolysis

Photolysis is degradation caused by light: a drug molecule absorbs a packet of light energy (a photon) and that energy snaps a bond or kicks off a reaction, much as sunlight fades a curtain or yellows old newspaper. It is the reason so many medicines come in brown bottles or foil blisters.

Only light that the molecule actually absorbs can do harm, so colour and chemical structure matter: molecules with conjugated double bonds, aromatic rings or certain functional groups absorb ultraviolet or visible light and are at risk. The absorbed energy may break the molecule directly, or it may sensitise oxygen and trigger oxidation, so photolysis and oxidation often work hand in hand.

Protection is mostly physical: opaque, amber or aluminium packaging, secondary cartons, and storage away from sunlight; sometimes a UV-absorbing excipient or opacifier is added. Light-sensitive infusions (for example nitroprusside or some vitamins) may need wrapping or amber giving-sets during administration, not just on the shelf.

The ICH photostability guideline (Q1B) requires deliberate light-exposure testing to find out whether a product even needs such protection — a reminder that photostability must be demonstrated, not assumed.