US FDA
21 CFR 211.166 - Stability Testing
Plain-language explanation, inspection context and common violation patterns.
What it says
A written testing programme designed to assess the stability characteristics of drug products must be established, including sample size and testing intervals based on statistical criteria, storage conditions, reliable and meaningful test methods, and testing of the drug product in the same container-closure system as marketed.
Paraphrased for plain-language clarity. Always verify against the current published regulation text.
What it means in practice
- Stability testing must use the actual commercial container-closure system, not a proxy or simplified packaging configuration, since the container itself affects product stability.
- The ongoing stability programme, not just initial registration stability, is a continuing regulatory obligation for the life of the marketed product.
- Statistical rigor in sample size and testing interval design is expected to be genuine, not simply defaulted to whatever a generic template specifies.
What FDA inspectors look for
- Whether ongoing annual stability batches are genuinely representative of commercial production, rather than selectively chosen from best-performing batches.
- Whether out-of-trend results, results within specification but showing an adverse trajectory, are investigated before they become out-of-specification.
- Whether stability protocols specify the statistical and analytical approach in advance, rather than the approach being selected after data is already in hand.
Most common violation
An adverse stability trend that stays within specification for multiple consecutive testing intervals without ever triggering a formal investigation into whether the registered shelf life remains valid.
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