21 CFR 211.166
Stability testing
21 CFR 211.166 is the regulation titled “Stability testing” in 21 CFR Part 211, Current Good Manufacturing Practice for Finished Pharmaceuticals. Since 2021, 166 FDA warning letters have cited it.
- Letters citing it
- 166 since 2021
- Jan 1 to Oct 30, 2026
- 28 against 33 on the same dates in 2025
- Most often cited with
- 21 CFR 211.84, in 110 letters
- Closed out
- 4% of these letters have a published close-out
Letters citing it, by year issued
Most are drug letters (148 of 166). 2026 runs to Oct 30, 2026. Pick a year to see its letters.
What FDA writes when it cites it
The finding headings that cite 21 CFR 211.166 most often, as written in the letters.
- 44 lettersYour firm failed to establish and follow an adequate written testing program designed to assess the stability characteristics of drug products and to use results of stability testing to determine appropriate storage…
- 13 lettersYour firm failed to establish and follow an adequate written testing program designed to assess the stability characteristics of drug products (21 CFR 211.166(a))
- 6 lettersYour firm failed to establish an adequate written testing program designed to assess the stability characteristics of drug products and to use results of stability testing to determine appropriate storage conditions and…
The regulation
Read the text of 21 CFR 211.166 (eCFR, as of Sep 1, 2026)
(a) There shall be a written testing program designed to assess the stability characteristics of drug products. The results of such stability testing shall be used in determining appropriate storage conditions and expiration dates. The written program shall be followed and shall include:
(1) Sample size and test intervals based on statistical criteria for each attribute examined to assure valid estimates of stability;
(2) Storage conditions for samples retained for testing;
(3) Reliable, meaningful, and specific test methods;
(4) Testing of the drug product in the same container-closure system as that in which the drug product is marketed;
(5) Testing of drug products for reconstitution at the time of dispensing (as directed in the labeling) as well as after they are reconstituted.
(b) An adequate number of batches of each drug product shall be tested to determine an appropriate expiration date and a record of such data shall be maintained. Accelerated studies, combined with basic stability information on the components, drug products, and container-closure system, may be used to support tentative expiration dates provided full shelf life studies are not available and are being conducted. Where data from accelerated studies are used to project a tentative expiration date that is beyond a date supported by actual shelf life studies, there must be stability studies conducted, including drug product testing at appropriate intervals, until the tentative expiration date is verified or the appropriate expiration date determined.
(c) For homeopathic drug products, the requirements of this section are as follows:
(1) There shall be a written assessment of stability based at least on testing or examination of the drug product for compatibility of the ingredients, and based on marketing experience with the drug product to indicate that there is no degradation of the product for the normal or expected period of use.
(2) Evaluation of stability shall be based on the same container-closure system in which the drug product is being marketed.
(d) Allergenic extracts that are labeled “No U.S. Standard of Potency” are exempt from the requirements of this section.
[43 FR 45077, Sept. 29, 1978, as amended at 46 FR 56412, Nov. 17, 1981]
Text from eCFR as of Sep 1, 2026.
Cited alongside it
Regulations that appear in the same letters as 21 CFR 211.166.
- 21 CFR 211.84Testing and approval or rejection of components, drug product containers, and closures110 letters cite both
- 21 CFR 211.100Written procedures; deviations105 letters cite both
- 21 CFR 211.22Responsibilities of quality control unit98 letters cite both
- 21 CFR 211.165Testing and release for distribution89 letters cite both
- 21 CFR 211.160General requirements52 letters cite both
- 21 CFR 211.192Production record review47 letters cite both
- 21 CFR 211.188Batch production and control records37 letters cite both
- 21 CFR 201.128Meaning of “intended uses”36 letters cite both
Latest letters citing it
The 3 letters citing 21 CFR 211.166 that FDA posted most recently.
- DrugsView Guangdong Bangze Medicine Co., Ltd.OTC drug productsIssuedOct 26, 2026PostedNov 3, 2026
- DrugsView Sanrace Biotechnology Co., Ltd.Over-the-counter (OTC) drug productsIssuedOct 22, 2026PostedOct 27, 2026
- DrugsView TSP Solutions Pte. Ltd.OTC sunscreen drug productsIssuedOct 22, 2026PostedOct 27, 2026
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Counts include every citation in a letter, as the Letters filter does. How this database is built.