solid-state stability

A good solid medicine should be the same on the day a patient takes it as it was the day it was made. Solid-state stability is exactly that quality: the resistance of a solid drug form to physical or chemical change during manufacturing, storage and shelf life, despite heat, light, humidity and time.

Stability has two faces. Chemical instability means the molecule itself degrades — by hydrolysis, oxidation, photolysis or reaction with an excipient — losing potency and forming impurities. Physical instability means the molecule survives but its solid form changes: a metastable polymorph converts, an amorphous form recrystallizes, a hydrate gains or loses water, crystals change habit, or particles aggregate, any of which can alter dissolution and bioavailability without changing the assay.

Solid-state stability is probed in preformulation by stressing the drug under elevated temperature, humidity and light and watching for both chemical degradation (by chromatography) and form changes (by X-ray powder diffraction and differential scanning calorimetry). The findings drive choices of salt, polymorph, packaging, desiccant and storage conditions, and feed the formal ICH stability program that sets the product's shelf life. The honest point is that physical changes are easy to overlook because potency can look fine while dissolution quietly drifts.