Accepted answer
503A and 503B are two different exemptions from the same set of federal requirements, and they buy different things at different prices. The shortage list mattered because of a single phrase in the statute — "essentially a copy of a commercially available drug product" — and what "commercially available" is taken to mean.
The two exemptions side by side
| 503A traditional compounding pharmacy | 503B outsourcing facility |
|---|
| Primary regulator | State board of pharmacy; FDA involvement is largely reactive | Registers with FDA and is inspected on a risk-based schedule |
| Prescription requirement | Patient-specific prescription required for an identified individual | May compound for office stock without a patient-specific prescription |
| CGMP | Exempt | Must comply with CGMP |
| Permitted bulk drug substance | Must be the subject of a USP/NF monograph, be a component of an FDA-approved drug, or appear on the 503A bulks list | Must appear on the 503B bulks list, or be on FDA's drug shortage list |
| Copies of approved drugs | May not compound a drug that is essentially a copy of a commercially available product | Same restriction, worded slightly differently |
| Adverse-event reporting | Not federally mandated | Mandatory to FDA |
| Product reporting | None | Twice-yearly report of everything compounded |
| Labelling | Exempt from adequate-directions-for-use requirements | Must carry a defined federal label set, including a statement that it is a compounded drug |
| Sterility testing | Only as USP chapters and the assigned dating require | Release testing expected as part of CGMP |
| Practical scale | Per-prescription batches | Industrial batches, often thousands of units |
Note the asymmetry in the bulk-substance row, because it is the whole answer to your third question. A 503A pharmacy has three independent doorways to a permitted API. A 503B facility has one list, plus the shortage clause.
Why a shortage created a permission
Both exemptions forbid compounding something that is essentially a copy of an approved commercial product. That restriction exists so compounding stays a practice of pharmacy for individual clinical needs rather than a parallel unapproved manufacturing industry. The operative question then becomes whether the approved product is "commercially available" — and a product that FDA has listed as being in shortage is not treated as commercially available. Listing therefore suspended the copy restriction. It did not create a new authorisation to compound; it removed the objection that otherwise blocked it.
That is a fragile foundation and everyone in the industry knew it. It was a temporary consequence of a supply fact, and it evaporated the moment the supply fact changed.
Resolution closed the doors on different dates
No, not simultaneously — and the staggering is deliberate. When FDA declared the tirzepatide shortage resolved, it announced it would not take action against 503A compounding until mid-February 2025 and against 503B facilities until mid-March 2025. Semaglutide followed the same pattern a couple of months later, with a 503A date in late April 2025 and a 503B date in late May 2025. 503B got the longer runway because outsourcing facilities have manufactured inventory and downstream clinic contracts to unwind; a 503A pharmacy makes to order.
The important structural consequence: post-resolution, the two categories are not in the same position. For 503B the shortage clause was the only route to the API, because these molecules are not on the 503B bulks list — so that route closed completely. For 503A the argument continues, because a 503A pharmacy can still claim a doorway via "component of an FDA-approved drug". But that doorway only opens for the substance in the form present in the approved drug, which is exactly where the salt-form problem lives, and it does nothing at all about the copy restriction, which came back into force on resolution.
What the sites still selling are relying on
Usually one of four things, in descending order of legitimacy:
- A genuine clinical-difference claim. The copy restriction has an exception where a compounded preparation is changed for an identified individual patient and the prescriber determines the change produces a significant clinical difference for that patient. This is a real exception. It is also narrow: a different concentration for convenience, or adding a vitamin, has been widely questioned as satisfying it, and "our version is dosed in units" is not a clinical difference.
- Personalised dosing that is a difference in name only. Very common. A grid of fixed doses offered to everybody is not individualisation, whatever the intake form says.
- Not a pharmacy at all. A research-chemical supplier selling a lyophilised powder labelled research use only is not compounding and is not claiming to; it is a different legal universe with no pharmacy oversight, no patient relationship, and no sterility or content guarantee. That material is not approved for human use.
- Offshore. Outside US jurisdiction entirely, and therefore outside every protection described in the table above.
How to check a claim: a 503B facility appears on FDA's published registered-outsourcing-facility list, and both categories hold state licences you can verify with the board of pharmacy in the state you are in, not just the state they are in. A pharmacy that will not name its state licence number has answered the question.
edited 12 Jun 2025 by u100_marks — reworded for clarity after a comment
2The three-doorways-versus-one-list framing finally made the 503B collapse make sense to me. – coldpack_88 6 months ago Also worth checking the non-resident pharmacy licence for your own state. Plenty ship where they are not licensed. – meniscus_film 4 months ago Dates match the notices I got from two different pharmacies, roughly a month apart by category. – ilaria_bertone 9 months ago add a comment