The short answer: a shortage listing, not a special GLP-1 rule
Compounded semaglutide and compounded tirzepatide were never approved by the FDA, and no rule was ever written that singled them out for permission. What made them lawful in the United States was a general provision of drug law: a compounding pharmacy or an outsourcing facility may make what amounts to a copy of an approved drug when that drug sits on the FDA drug shortage list. Semaglutide injection and tirzepatide sat on that list for roughly two years. The copies followed. That is the whole mechanism.
The corollary caught patients and prescribers off guard. Nothing about the compounded product itself had to change for its legal status to change. When FDA determined that supply met or exceeded national demand and removed the products from the shortage list, the exemption that had covered wide-scale compounding of copies switched off by operation of the statute, not because a new prohibition was written. FDA declared the tirzepatide shortage resolved in October 2024, reopened the question under litigation pressure, and reaffirmed it that December. It declared the semaglutide injection shortage resolved in February 2025.
This article explains the mechanism rather than the headline, because the mechanism is what stays true. The positions here have moved repeatedly, driven by agency determinations, court filings and temporary enforcement discretion. Anyone making a decision that turns on the current status should read the FDA drug shortage database and FDA's compounding guidance directly.
What compounding is, and what the word does not promise
Compounding is the preparation of a medicine by a licensed pharmacist or physician, by combining, mixing or altering ingredients for a particular patient. It predates the modern approval system and was left in place deliberately, because some patients cannot take an approved product as manufactured: an excipient allergy, a strength that does not exist commercially.
The critical feature is what compounded products skip. There is no new drug application, no FDA review of safety or effectiveness, no agency-reviewed prescribing information, and no premarket assessment of manufacturing quality. Sections 503A and 503B do not approve anything; they carve out exemptions. Legally compounded is a statement about regulatory category, not about quality: it says nothing about whether a given vial contains the labelled amount of the labelled molecule, whether it is sterile, or whether it will still be potent at the end of its stated shelf life.
503A: the traditional pharmacy lane and its conditions
Section 503A covers traditional compounding by a licensed pharmacist or physician. The exemption applies only where the product is compounded for an identified individual patient on the basis of a valid prescription, and where the prescriber has determined that the compounded product is necessary for that patient. It is a patient-by-patient permission, not a licence to manufacture.
The second condition is the ingredient. A bulk drug substance may be used under 503A only if it is the subject of a United States Pharmacopeia or National Formulary monograph, or is a component of an FDA-approved drug product, or appears on FDA's 503A bulk drug substances list. It must arrive with a certificate of analysis from a registered facility. Semaglutide base cleared this condition because it is a component of approved products.
The third is the one this entire story turns on: a 503A pharmacy may not compound a drug that is essentially a copy of a commercially available drug product, meaning the same active ingredient in a comparable dosage form, route and strength. The statute leaves one narrow opening, where the prescriber determines that the compounded product will produce a significant difference for the identified patient and documents that on the prescription. 503A pharmacies are also exempt from federal manufacturing standards; sterile compounding is instead governed by compendial standards enforced by state boards of pharmacy.
503B: outsourcing facilities, and why they mattered most here
Section 503B was created by the Drug Quality and Security Act of 2013, in direct response to a 2012 fungal meningitis outbreak traced to contaminated compounded steroid injections that killed dozens of people. It created a category sitting between a pharmacy and a manufacturer: the registered outsourcing facility registers with FDA, complies with current good manufacturing practice, submits to inspection, and may compound without a patient-specific prescription and ship to clinics as office stock. That is the lane that made compounded GLP-1 products available at scale through clinics and telehealth prescribing.
Its ingredient rule is tighter than 503A's. An outsourcing facility may use a bulk drug substance only if that substance appears on FDA's 503B bulk drug substances list, or if the drug is on FDA's drug shortage list. There is no monograph or component-of-an-approved-drug route. That second clause is why the shortage listing did so much work: for outsourcing facilities it was the direct and only statutory basis for using semaglutide or tirzepatide bulk at all. Section 503B carries its own prohibition on producing essentially a copy of an approved drug, with an explicit carve-out for drugs on the shortage list, so both lanes hung on the same hook.
Why commercially available is the load-bearing phrase
The copy prohibition in both sections is defined against whether the approved drug is commercially available, and a drug appearing on the shortage list is not treated as commercially available for that purpose. This is worth stating precisely, because it is routinely described backwards. Shortage listing does not grant a permission to compound. It removes a prohibition that would otherwise apply, and when the listing ends the prohibition returns to its default state without anyone having to act.
How a product reaches the list is equally mechanical. Manufacturers must notify FDA of interruptions in supply of certain drugs, and FDA maintains the resulting list. A shortage is resolved when the agency judges that supply from all manufacturers meets or exceeds national demand. That is a judgement about aggregate national supply, not a guarantee that every pharmacy has stock on a given morning, which is why patients kept encountering local gaps after each resolution while the legal basis for compounding had already lapsed.
FDA did soften the transition, announcing periods after each resolution during which it did not intend to take enforcement action, shorter for 503A pharmacies and longer for 503B facilities, on the order of two and three months respectively. Those should be read correctly. Enforcement discretion is a statement about what an agency intends to pursue. It is not a legal safe harbour, it confers no rights, and it can be withdrawn.
The salt forms were never covered, shortage or no shortage
Approved semaglutide products contain semaglutide as the free base. A substantial share of the material offered to compounders and to the grey market was not that. It was semaglutide sodium or semaglutide acetate, salt forms of the molecule, distinct chemical entities with a distinct counterion, molecular weight and handling behaviour even though the peptide sequence is the same.
Run those through the ingredient conditions and they fail everywhere. There is no compendial monograph for either. Neither is a component of any FDA-approved drug product, because the approved products use the base. Neither appears on the eligible bulk drug substances lists. FDA reviewed the salt forms and placed them in the category reserved for substances raising significant safety concerns, on the basis that they were not shown to be the same active moiety used in the approved products.
This is the most misunderstood point in the subject. The shortage listing switched off the copy prohibition. It never converted an ineligible bulk drug substance into an eligible one. Products built on salt forms were outside both exemptions the entire time, including at the height of the shortage when everything else about the activity was permitted, and the distinction is invisible to a patient because a vial labelled semaglutide does not disclose which form went into it.
What happened after the shortages were declared resolved
The resolutions were contested immediately. A trade association representing outsourcing facilities sued FDA over the tirzepatide determination, arguing the agency had acted on inadequate evidence of restored supply. FDA reconsidered, gathered further information, and reaffirmed the resolution in December 2024. Federal courts declined to block the agency, and a similar pattern followed for semaglutide in 2025. The manufacturers separately pursued litigation against compounders and marketers of compounded copies.
The economics shifted at the same time. Both manufacturers opened lower-priced cash-pay routes to their approved products during 2025, removing part of the price gap that had driven demand toward compounded versions.
What persisted were the workarounds: products offered at non-standard strengths, on the argument that a strength which does not exist commercially cannot be a copy; combination products, most commonly semaglutide with an added vitamin; and a research-use-only market in which vials are sold labelled not for human consumption. FDA's guidance is explicit that a change in strength or an added ingredient does not by itself take a product outside the copy prohibition, and research-use-only labelling is a disclaimer of liability rather than a regulatory pathway. The statutory test is a documented, patient-specific determination of significant difference by the prescriber, not a product category invented by a seller.
What the quality of a compounded product actually rests on
With no premarket review, the quality of a compounded injectable rests entirely on the compounder and, where it exists, on testing the compounder or seller commissions and chooses whether to publish. Third-party testing arranged by a party with a commercial interest in the result is better than nothing and worse than an inspected quality system. For 503A pharmacies there is no federal manufacturing standard at all.
The failure modes documented during this period were not hypothetical. FDA received adverse event reports involving compounded semaglutide that included overdoses arising from measurement error, where patients drawing from a vial misread syringe unit markings against a volume, an error the approved pen devices are engineered to prevent. Separately, health authorities in several countries documented falsified semaglutide pens entering legitimate distribution in 2023, some of which contained insulin and caused hypoglycaemia. Underneath both is the fact that peptides are demanding to formulate and to keep: they aggregate, oxidise and adsorb to container surfaces, and multi-dose presentations require a preservative system and a defensible beyond-use date.
Why these molecules are drugs and not biologics, and why that decides the rules
One structural fact sits underneath everything above. United States law draws the line between a drug and a biological product at protein size: a polymer of more than forty amino acids is a protein and is regulated as a biologic under a licence, while shorter peptides remain drugs regulated under new drug applications. Insulin crossed that line during the 2020 transition and left the small-molecule framework behind.
Semaglutide is a thirty-one amino acid analogue of GLP-1, modified with a fatty diacid chain that drives albumin binding and gives it a half-life on the order of a week. Tirzepatide is a thirty-nine amino acid dual GIP and GLP-1 receptor agonist built on a similar acylation strategy. Both sit on the drug side of that boundary by a margin of a few residues, and that accident of length is why this article is about 503A and 503B at all. Had either been slightly longer it would be a licensed biologic, and these bulk substance pathways would not apply in the same way.
What the approved products were actually shown to do
It is worth anchoring what the copies were copying, because the evidence base belongs to the approved products and does not transfer. In the 68-week phase 3 obesity trial of semaglutide at 2.4 mg once weekly, mean body weight fell by close to fifteen percent from baseline against a small placebo change. In the 72-week phase 3 obesity trial of tirzepatide, mean reductions rose with dose and reached around a fifth of body weight at the highest dose studied. The cardiovascular outcomes trial of semaglutide 2.4 mg, in people with established cardiovascular disease and overweight or obesity but without diabetes, reported roughly a twenty percent relative reduction in major adverse cardiovascular events.
None of those findings were generated with compounded material, and none are evidence about a compounded product's potency, purity or stability. A compounded copy inherits the pharmacology of the molecule only if it actually contains the molecule at the stated amount. That conditional is the entire risk, and removing it is what the approval system exists to do.
How to check the current position instead of trusting this page
Shortage status is assigned per drug product and presentation, not per molecule, and it can move in either direction. The FDA drug shortage database is the authoritative record. Ingredient eligibility is a separate check, answered by FDA's bulk drug substances lists and the category assignments that accompany them; a substance can be nominated, evaluated and moved between categories. Anything asserting that a bulk form is eligible should be traced to those lists, not to a supplier's characterisation of them.
None of this transfers across borders. The United Kingdom handles unlicensed preparations through a specials regime under the MHRA rather than anything resembling 503A and 503B, European rules vary by member state, and both the MHRA and the European Medicines Agency issued their own alerts about falsified pens on timelines unrelated to the American shortage listings.
The honest position for a reference site is therefore this. We can describe with confidence what makes compounding permissible, what turns it off, and why the salt forms were always outside it. We cannot certify today's status, because it has changed several times already and is subject to determinations and court decisions that postdate anything written here. Check the regulator.