Definition: Drug stability means the ability of the pharmaceutical dosage form to maintain the physical, chemical, therapeutic and microbial properties during the time of storage & usage by patients.
Measured by the rate of changes that take place in pharmaceutical dosage forms.
Expiry date: means that drug can not be used after this date because the conc. of drug decreased & become lower than therapeutic conc.
Instability may cause Undesired change in performance; i.e. dissolution/bioavailability