Detailed definition
Section 503B of the FDA Modernization Act, added by the Drug Quality and Security Act of 2013 in response to the New England Compounding Center fungal meningitis outbreak, created a category of "outsourcing facilities." 503B facilities register with the FDA, follow cGMP standards similar to manufacturers, and are subject to FDA inspection. They can compound batches of medications without patient-specific prescriptions for office stock or hospital use. 503B compounding still does not yield FDA-approved products — it is regulated production of compounded medications. Some HRT products (notably some estradiol/testosterone pellet preparations) are produced at 503B facilities for clinic stock.
Why it matters in menopause
503B compounding is a step up in quality assurance from 503A but is not equivalent to FDA approval. The relevant context for HRT is that even some "office-stock" pellets and creams come from 503B facilities — the regulatory layer is better than 503A but the underlying safety questions about pellet doses and supraphysiologic levels remain.
Related terms
Sources
External references: Wikipedia.