GLP-1 Compounding Glossary: Key Terms Explained
The regulatory and clinical debate around compounded GLP-1 drugs relies on a vocabulary that is not intuitive. This glossary defines the key terms used in coverage, prescribing, and policy discussions.
503A pharmacy
Definition: A state-licensed compounding pharmacy that prepares medications for individual patients based on a valid prescription, under §503A of the Federal Food, Drug, and Cosmetic Act.
Key rules for GLP-1 compounding under 503A:
- Requires a valid prescriber-patient relationship
- Can compound drugs that are commercially available, provided the prescriber documents a legitimate clinical reason (see Significant difference below)
- Regulated primarily by state pharmacy boards; FDA oversight is indirect (mainly through enforcement for violations)
- Cannot produce compounded drugs for speculative sale — each batch must correspond to specific patient prescriptions
Current status (May 2026): 503A is the active legal pathway for GLP-1 compounding. Most remaining GLP-1 telehealth providers (Henry Meds, Mochi, Eden, Noom via Tailor Made Compounding) use 503A-licensed pharmacies. For a detailed comparison of the two pathways, see 503A vs 503B pharmacies explained.
503B outsourcing facility
Definition: An FDA-registered facility that can compound larger quantities of drug products — including without patient-specific prescriptions — under Current Good Manufacturing Practice standards, regulated under §503B of the FDCA.
Why it mattered for GLP-1s: During the 2021–2024 shortage, FDA added semaglutide and tirzepatide to the drug shortage list. This allowed 503B facilities to compound GLP-1 bulk substances (API) at scale under the shortage exemption. This was the mechanism behind large-scale telehealth GLP-1 compounding.
Current status (May 2026): FDA removed semaglutide from the shortage list in February 2025 and tirzepatide in October 2024. 503B compounding authority for both drugs closed by May 22, 2025. The FDA's April 30, 2026 proposal would permanently exclude semaglutide, tirzepatide, and liraglutide from the 503B bulk substances list. For the full regulatory timeline, see the compounding cliff timeline.
Drug shortage exemption
Definition: A provision allowing 503A and 503B compounders to prepare copies of commercially available drugs when those drugs are on the FDA's official drug shortage list. Once a drug is removed from the shortage list, the exemption ends.
Timeline for GLP-1s:
- Tirzepatide shortage declared: 2022
- Semaglutide shortage declared: 2022
- Tirzepatide shortage resolved: October 2, 2024
- Semaglutide shortage resolved: February 21, 2025
- 503B enforcement-discretion window for semaglutide compounding closed: May 22, 2025
Significant difference (§503A(b)(1)(D))
Definition: A provision allowing a 503A pharmacy to compound a drug that is essentially a copy of a commercially available drug if the prescriber documents a clinical reason why the commercially available version cannot meet the specific patient's needs. Examples include: a dose not available commercially, an allergen-free formulation, an alternative delivery route, or an added ingredient.
How GLP-1 compounders use it: Many remaining compounders argue that a personalised dose schedule, B12 or L-carnitine additives, or a specific formulation constitutes a "significant difference" from Wegovy or Zepbound.
FDA's position: The FDA has stated that compounded products that are "essentially copies" of commercially available drugs — without a genuine clinical distinction — do not qualify for this provision. The agency has issued warning letters to compounders it determined did not meet the significant difference standard.
Bulk drug substance
Definition: The active pharmaceutical ingredient (API) before it is formulated into a finished drug product. Compounders source bulk semaglutide or tirzepatide API, then prepare patient-specific formulations.
Why this matters: API sourcing is not equivalent to using the finished drug. Bulk semaglutide exists in multiple salt forms (see Salt forms below), and the quality, purity, and stability of bulk API varies by source. FDA does not pre-approve bulk API for compounding use; quality depends on the compounder's sourcing and testing practices.
Salt forms (semaglutide base vs semaglutide sodium)
Definition: The same active molecule (semaglutide) exists in different salt forms: semaglutide (free base) and semaglutide sodium. These have different molecular weights.
Why it matters: Commercial Wegovy and Ozempic contain semaglutide free base. Some compounders sourced semaglutide sodium API. Because the molecular weights differ, a "dose" specified in mg may deliver a different amount of active semaglutide depending on which salt form was used. The FDA identified this as a source of potential dosing error in adverse-event reports and warning letters for compounded GLP-1 products.
SNAC
Definition: Sodium N-(8-[2-hydroxybenzoyl]amino)caprylate. An absorption enhancer used in oral semaglutide (Rybelsus, oral Wegovy) to allow the peptide to be absorbed from the stomach.
Mechanism: SNAC creates a transient local alkaline microenvironment in the gastric mucosa, reducing peptide degradation and enhancing absorption via transcellular transport. Without SNAC, oral semaglutide bioavailability approaches zero.
Relevance to compounding: Compounded oral semaglutide preparations may not include SNAC, or may include it at different concentrations. Without appropriate SNAC formulation, an oral compounded semaglutide product may deliver little to no active drug even if the label dose is correct.
Bioequivalence
Definition: The FDA standard for determining whether a generic drug delivers the same active ingredient at the same concentration and rate as the reference listed drug. Bioequivalence testing is required before a generic can be approved.
Relevance to compounded GLP-1s: Compounded GLP-1 medications are not generics. They have not been through bioequivalence testing. A compounded semaglutide preparation is not equivalent to Wegovy or Ozempic under FDA standards — it is prepared with the same active ingredient but without the formulation validation, stability testing, or manufacturing process validation that FDA-approved products undergo. For a clinical-level comparison, see our article on compounded vs brand semaglutide.
Warning letter (FDA)
Definition: An official communication from the FDA to a company or individual identifying a significant violation of regulations the FDA enforces. A warning letter is a formal regulatory action but not a shutdown order or recall.
Relevance to GLP-1 compounding: FDA issued approximately 50+ warning letters to GLP-1 compounders and telehealth platforms in September 2025 and March 2026. Warning letter violations for GLP-1 platforms fell into two categories: safety-related (sterility, dosing errors, salt form issues) and marketing-related (false equivalence claims, misleading advertising). A warning letter recipient must respond within 15 business days and correct violations; failure to correct can escalate to injunctions, seizures, or criminal referral.
Adverse event report (MedWatch)
Definition: A report filed with the FDA (through its MedWatch system) by a patient, prescriber, or company reporting a negative outcome associated with a medication. Reports are submitted voluntarily by the public and mandatorily by manufacturers.
Relevance to compounded GLP-1s: FDA highlighted a concentration of adverse event reports involving compounded GLP-1s in 2023–2025, including hospitalizations and serious complications, noting that some reports described dosing errors related to salt-form confusion or incorrect concentration labelling. Adverse event reports do not establish causation but inform FDA enforcement priorities.
For the full regulatory timeline, see the compounding cliff timeline. For the live tracker of which compounders have received warning letters, see the FDA warning letters tracker.