FDA Warning Letter: Deficiencies in Impurity Testing, Stability, OOS Investigations and Temperature Control

The U.S. Food and Drug Administration (FDA) has issued a Warning Letter dated 8 September 2026 to a manufacturer of over-the-counter (OTC) drug products in Chatsworth, California.

Following an inspection from 31 March to 3 April 2026, the FDA identified significant violations of Current Good Manufacturing Practice (CGMP) requirements under 21 CFR Parts 210 and 211. The main deficiencies concerned impurity testing, stability studies, OOS investigations and warehouse temperature control.

Inadequate Impurity Testing

The company had not established appropriate specifications for certain OTC products containing ingredients that may generate a potentially carcinogenic impurity through degradation or chemical reaction.

Despite this known risk, testing for the impurity was not included in release or stability testing. The FDA stated that at least one batch had been distributed without the required testing.

The company had also failed to assess a contamination event involving products manufactured with the same formulation, packaging and labelling for the United States and Canada. A customer had recalled products in Canada, but the company had not determined whether related batches had been distributed in the United States.

The company committed to discontinuing certain products, using a third-party laboratory and updating its specifications. The FDA considered the response inadequate because the company did not provide sufficient test results for products remaining on the U.S. market, identify all potentially affected batches or demonstrate the laboratory’s qualification.

Stability Programme Deficiencies: Inadequate Investigation of Viscosity OOS Results

The FDA found that the company’s stability programme did not use reliable and meaningful methods to assess product stability.

The FDA states: "Your firm failed to adequately investigate viscosity failures obtained during stability testing of distributed OTC drug products. Specifically, stability testing records documented [...] out-of-specification (OOS) results without adequate root cause determination, CAPA, or documented justification [...]".

Temperature Excursions in Warehouse Areas

The FDA also identified inadequate temperature monitoring and storage controls. In one example, a product was stored at temperatures exceeding both its labelled storage conditions and the company’s internal requirements.

A review of temperature data from April 2025 through March 2026 identified several periods above the applicable limits. The FDA noted that elevated temperatures may increase the risk of impurity formation. The company had also failed to include storage requirements on its Certificates of Analysis.

The FDA requested a retrospective review of temperature data covering all storage areas and OTC products. The company must assess the impact on product quality, safety and efficacy, provide a risk assessment for the affected batch and compare labelled storage conditions with recorded data across all buildings.

Quality Unit Authority and CGMP Consultant

The FDA recommended engaging a qualified CGMP consultant in accordance with 21 CFR 211.34. However, management remains responsible for resolving the deficiencies and ensuring sustained compliance.

The FDA also concluded that the quality unit had not been able to fully exercise its authority and responsibilities. The company must provide the quality unit with sufficient authority and resources to ensure the identity, strength, quality and purity of its products.

For further information, please see the complete FDA Warning Letter to kdc/one Chatsworth, Inc. on the FDA website.