stability
Stability is a medicine's ability to stay the same as the day it was made. Think of a carton of milk: fresh and good for a while, then slowly it sours. A tablet or injection is similar — it should keep its strength, look, and safety for the whole time stated on the box, and stability is the science of how well and how long it does so.
More precisely, a product is stable if it remains within its specification — the agreed limits for drug content, appearance, dissolution, microbial quality and breakdown products — across its claimed lifetime under defined storage conditions. Stability has several faces: chemical (the active molecule decomposing), physical (crystals changing, a suspension caking, a cream splitting), microbiological (preservative failing, contamination growing), and therapeutic (the dose still working as intended).
Stability is built in, not bolted on: it depends on the molecule's intrinsic reactivity, the formulation (pH, antioxidants, chelating agents), the container, and the storage environment (heat, light, moisture, oxygen). Studies under controlled and stressed conditions establish how fast a product changes, which fixes the shelf life and storage instructions.
An honest caveat: a product passing stability tests is only guaranteed within the labelled conditions and packaging. Repackaging, breaking the seal, freezing a refrigerated biologic, or leaving a bottle in a hot car can void those guarantees long before the printed date.
Aspirin tablets slowly hydrolyse to acetic acid and salicylic acid; an old bottle that smells of vinegar has lost both strength and quality, illustrating chemical instability you can literally smell.
Stability you can smell: aspirin's vinegar odour signals hydrolysis.
Stability is not just about the drug molecule. A perfectly intact active can still fail stability if the tablet hardens and stops dissolving, a suspension cakes, or a preservative degrades and lets microbes grow.