If you've used a compounded GLP-1, you've injected a medication that a pharmacy mixed from raw ingredients. It didn't come off the production line that makes the branded pens, and that difference shapes how the two are made, tested, and regulated. This guide follows the path from drug powder to the vial in your hand and compares the compounded and branded routes at each step.
What "compounding" actually means
Compounding is the practice of a licensed pharmacy combining or altering ingredients to make a medication for a patient. For GLP-1s, that means starting with the active pharmaceutical ingredient (API), the drug substance itself as a powder, and turning it into an injectable solution. Manufacturing an FDA-approved drug is a different activity. A manufacturer makes millions of identical, pre-tested units under federal approval. A compounder prepares a product that the FDA has never reviewed for safety, effectiveness, or quality before it reaches you.
Two kinds of facility are allowed to do this, and the difference matters:
- 503A pharmacies are state-licensed and compound against an individual prescription for a named patient. The state board of pharmacy does most of the inspecting, and they follow the U.S. Pharmacopeia (USP) standards for compounding, including USP <797> for sterile products. They are not required to follow the federal "current good manufacturing practice" (CGMP) rules that manufacturers must.
- 503B outsourcing facilities register directly with the FDA, can compound larger batches without a patient-specific prescription, and must meet full CGMP and risk-based FDA inspection. That puts them a step closer to a manufacturer, but they still aren't making an FDA-approved drug.
Step 1: Sourcing the active ingredient
A legitimate compounder can't just buy GLP-1 powder from anywhere. The API is supposed to be pharmaceutical-grade, come with a valid certificate of analysis documenting its identity and purity, and be made by an establishment registered with the FDA (under section 510 of the federal drug law). Reputable pharmacies send finished product to third-party labs and source raw material from FDA-registered suppliers to try to hold that line.
This is also where one of the biggest safety problems sits: the salt-form issue. The FDA has warned that "salt" versions of the drug substance, such as semaglutide sodium and semaglutide acetate, are different active ingredients than what's in the approved medications. The agency says it has no information showing these salts behave the same way in the body, and it is not aware of a lawful basis for using them in compounding. State pharmacy boards have echoed this. Mississippi's Board of Pharmacy, for example, told licensees in 2023 that compounding with semaglutide salts does not meet federal requirements. When you hear cautions about "research peptides" and grey-market powder, this is the core of it: what the label on the drum says, and where the powder really came from, decides almost everything. (Our guide on research peptides and GLP-1 medications covers the grey-market end of this in detail.)
Step 2: Determining the formula and dose
In a 503A setting, the formula starts with a prescription for a specific patient. The prescriber sets the drug, the concentration, and the dose. The pharmacy works out how to dissolve the API to that concentration in a sterile carrier, typically sterile water or a buffered saline solution, sometimes with other excipients to keep the peptide stable. Some compounded products come in concentrations or drug combinations (for example, paired with vitamin B12) that don't exist in any branded pen.
That flexibility is why compounding exists, and it's also where risk creeps in. Because a compounded vial isn't a fixed-dose pen, you often have to draw up a dose with a syringe, and the concentration may differ from what you're used to. The FDA has received reports of dosing errors, some serious enough to require hospitalization, tied to compounded injectable semaglutide, several traced to patients measuring the wrong amount. A branded pen removes that math; a compounded vial brings it back.
Step 3: How the pharmacy actually makes it
In a sterile 503A or 503B cleanroom operating under USP <797>, the physical process looks roughly like this:
- Dissolve the API powder into the sterile carrier at the target concentration.
- Sterile-filter the solution, usually through a 0.22-micron filter that removes bacteria.
- Aseptically fill the solution into sterile vials in a controlled, clean environment.
- Test representative units. Reputable operations send samples to third-party labs for potency (is the dose accurate?), sterility (is it free of microbes?), and endotoxin (is it free of the bacterial wall fragments that can cause fever and serious reactions?).
The gap between best and worst practice here is enormous, and the patient can't see it. A careful 503B facility under CGMP tests every batch; a cut-corner operation may test little and document less. The finished vial looks the same either way, so the source tells you more than the product's appearance.
How the branded pens are made
The branded medications, Ozempic and Wegovy (semaglutide, Novo Nordisk) and Zepbound and Mounjaro (tirzepatide, Eli Lilly), are made on a different scale and under a different rulebook.
Novo Nordisk produces semaglutide itself. The peptide backbone is made by yeast fermentation (recombinant biotechnology) at its facilities in Denmark and then chemically modified. Tirzepatide is likewise made to a fixed specification under federal manufacturing rules. Neither company is ordering powder from an outside supplier for these products.
The formula and dose aren't set per patient. They're fixed, studied in clinical trials, and approved by the FDA before a single pen is sold, and every batch is made and tested under federal good-manufacturing rules. Fill-finish is industrial and standardized: the drug goes into prefilled pens (or single-dose vials) at large plants, and each unit delivers the same metered dose. The full process, from drug substance to a finished, released pen, takes months, which is one reason branded shortages take months to resolve.
A branded pen is one of millions of identical units, each checked before it ships. A compounded vial is a small batch prepared after the prescription is written and checked, if at all, by the pharmacy and its labs, not by the FDA.
Where compounded GLP-1s stand now
Compounding of these drugs surged because the branded products were in official shortage, a status that under federal law opens a window for compounders. As the shortages resolved, that window narrowed sharply. FDA declared the tirzepatide shortage resolved on October 2, 2024 and reaffirmed that decision on December 19, 2024. It declared the semaglutide shortage resolved in February 2025. The FDA ended enforcement discretion for compounded tirzepatide in 2025 (503A pharmacies on February 18, 503B facilities on March 19), and ended enforcement discretion for compounded semaglutide on April 22, 2025 for 503A pharmacies and May 22, 2025 for 503B facilities. Since then, mass-produced "copies" of either drug are no longer permitted; what remains is patient-specific 503A compounding for a documented clinical need. This legal status has already shifted more than once, so re-check it against current, dated FDA guidance.
If you are using or considering a compounded GLP-1, the practical safeguard is to vet the pharmacy. Confirm it's a licensed 503A pharmacy or an FDA-registered 503B facility, ask what API it uses and whether it's the base drug (not a salt), and ask for third-party testing. Our guides on pharmacy sources for GLP-1 medications and how to verify a pharmacy is legitimate walk through how.
This article is general education, not medical advice. Regulations governing compounded GLP-1 medications are changing; confirm current status and any specific product with the FDA, your prescriber, or your pharmacist before acting.