Drug stability
Also known as: medication stability
Drug stability is the extent to which a medication retains its identity, strength, quality, and purity through storage and use. It determines expiration dating, beyond-use dating, and the storage conditions printed on every prescription label.
A stable drug product keeps its chemical composition, physical characteristics, microbiological quality, therapeutic effect, and toxicological profile within specification. Degradation happens through predictable routes: hydrolysis, oxidation, photolysis, and, for biologics, loss of three-dimensional protein structure. Loss of potency is only part of the concern, since some degradation products are inactive while others are irritating or toxic.
Four environmental factors drive most degradation — temperature, light, moisture, and air. That is why storage requirements are stated as defined ranges rather than vague instructions: refrigerated products belong at 2 to 8 °C, frozen products at −25 to −10 °C, and controlled room temperature is 20 to 25 °C. Amber vials protect photosensitive drugs, desiccant packets absorb moisture, and manufacturer packaging is often part of the stability data, which is why some tablets should not be repackaged into ordinary vials.
Two different dates apply to medications. The expiration date is set by the manufacturer from stability testing of the product in its original, unopened container. The beyond-use date applies once a product is repackaged, reconstituted, or compounded, and is always the earlier of the manufacturer expiration date or the interval established for that preparation. Reconstituted oral suspensions are the everyday example: the powder may be good for years, while the mixed suspension is good only for a set number of days and often requires refrigeration.
Technicians act on this information constantly — rotating stock, checking cold-chain storage, applying auxiliary labels, and calculating beyond-use dates at the point of dispensing. The PTCB Pharmacy Technician Certification Exam (PTCE) tests storage temperature ranges, the distinction between expiration and beyond-use dating, which products require refrigeration or protection from light, and how to identify a product that has been compromised.
Key takeaways
- Drug stability is a product's ability to retain identity, strength, quality, and purity over time.
- Temperature, light, moisture, and air are the main causes of degradation.
- Expiration dates apply to unopened manufacturer packaging; beyond-use dates apply after repackaging, reconstitution, or compounding.
- A beyond-use date can never extend past the manufacturer's expiration date.
- The PTCE tests storage temperature ranges and the expiration versus beyond-use date distinction.