USP <797> is the chapter of the United States Pharmacopeia that sets the minimum standards for preparing sterile medications — injectables, eye drops, and similar products — in a pharmacy. It covers the cleanroom environment, staff training and garbing, air quality, cleaning, and how long a preparation may be used before it is discarded. State boards of pharmacy adopt it, which is what makes it enforceable for a traditional 503A compounding pharmacy.
It matters for compounded GLP-1s because those are sterile injectables. A 503A pharmacy is not required to follow the FDA's current good manufacturing practice (CGMP) rules that drug manufacturers and 503B outsourcing facilities must meet; USP <797> is the standard it follows instead. Asking a pharmacy whether it complies with USP <797> — and whether the state board has inspected it — is one of the most concrete legitimacy checks available. See talking to a pharmacist about compounded medications and how compounded GLP-1 medications are made. The FDA notes that drugs from 503B outsourcing facilities are subject to CGMP requirements while those compounded by state-licensed pharmacies under section 503A are not, and that state boards of pharmacy oversee those pharmacies (FDA (external link)). USP's own FAQ on the chapter describes it as minimum standards and says USP has no role in enforcing it, which falls to regulators such as state boards of pharmacy (USP <797> FAQs (external link)).