Most coverage of compounded GLP-1 products argues about legality, purity or price. The recurring practical problem is none of those. It is arithmetic.
This page sets out where that arithmetic comes from, using the approved labels themselves, and what FDA has recorded about the results. It does not compare sellers and it does not publish a conversion.
The approved class removes the calculation, and it took real effort to do it
Read the dosage-forms section of every approved GLP-1 injection in turn and a design principle appears that no single label states.
The person injecting never measures anything. The dose is preset, and the way manufacturers achieve that is by varying the concentration so the volume can stay fixed.
| Product | Presentations | What varies | What stays constant |
|---|---|---|---|
| Ozempic pens | 2 mg/3 mL, 4 mg/3 mL, 8 mg/3 mL | Concentration: 0.68, 1.34, 2.68 mg/mL | One device, one injected volume per click |
| Zepbound / Mounjaro single-dose pens and vials | 2.5 mg to 15 mg | Concentration: 5 mg/mL to 30 mg/mL | 0.5 mL, every dose |
| Zepbound / Mounjaro multi-dose vials | Six strengths | Concentration: 4.17 to 25 mg/mL | 0.6 mL, every dose |
| Wegovy pens | Five doses | Concentration, across two fill volumes | 0.5 mL or 0.75 mL, preset per pen |
The tirzepatide line is the clearest case. A six-fold range of drug per dose — 2.5 mg to 15 mg — is delivered at one volume, because the manufacturer formulates six different concentrations to keep it there. That is not a packaging convenience. Manufacturing, stability-testing and labelling six concentrations of one molecule is expensive, and the thing it buys is that nobody downstream has to divide.
Ozempic makes the same point with a subtlety worth noticing: three pens that look alike contain 0.68, 1.34 and 2.68 mg/mL. Even within one brand, in one device, the concentration is not a constant a reader can assume.
What a compounded vial does instead
A compounded preparation is typically supplied as a vial at a single concentration, with a syringe. A different dose therefore means a different volume drawn, which is a multiplication and a division performed by hand, on a scale, under a light, possibly at speed.
That is the step the entire approved class is built to avoid. It is also precisely the step FDA names. Its page on unapproved GLP-1 drugs, current as of 2026-09-01, describes
"multiple reports of adverse events, some requiring hospitalization, that may be related to dosing errors associated with compounded injectable semaglutide products"
and identifies two mechanisms:
"patients measuring and self-administering incorrect doses of the drug, and in some cases, health care professionals miscalculating doses."
The second half of that sentence is the one that gets least attention. This is not only a patient-comprehension problem; FDA records professionals miscalculating as well.
As of 2026-05-31 the agency had received 990 adverse event reports for compounded semaglutide and more than 730 for compounded tirzepatide. Those totals cover all reported adverse events, not dosing errors alone, and a report establishes a temporal association rather than a cause. They are still the only public count there is.
The word "units", and why it is the sharpest edge in the whole problem
Compounded doses are frequently expressed in "units", because the syringe supplied is an insulin syringe and that is what its markings say.
A unit is not an amount of GLP-1. It is a mark on a barrel calibrated for a different drug. A U-100 insulin product is 100 units per millilitre — HUMULIN R's label states it plainly, "Injection: 100 units/mL (U-100)" — so one mark is 0.01 mL of liquid.
That is the whole of it: the mark measures volume. What the volume contains depends entirely on the concentration of the preparation in the vial, which is set by whoever compounded it and is not standardised across the market. Two vials with identical labelling in units can hold different amounts of drug per mark if their concentrations differ.
A pen has no equivalent failure mode, because a pen's dial is calibrated against the concentration sealed inside it.
What the reporting database already showed, on a different product
This site has already published the adverse-event finding that sits alongside all of this. In FAERS, the most-reported item for tirzepatide is not a symptom at all but a report about getting the dose wrong, and the size of the gap against semaglutide is set out with its control on our tirzepatide review. That page covers the approved products and the counting problem behind the figures; it is not restated here.
What this page adds is the mechanism on the compounded side, where FDA has named the cause in its own words rather than leaving it to be inferred from a reporting term.
Doses the approved labels do not contain
FDA records one further category, and it is not a measurement error. The agency has received reports of
"patients prescribed compounded semaglutide or tirzepatide products in doses beyond what is in the FDA-approved drug label"
covering more product per dose, more frequent administration, and faster titration than the approved schedules. The approved schedules themselves, all twelve of them, are set out side by side on our titration schedules page, where every label gives tolerability rather than efficacy as the reason for escalating slowly.
A compounded preparation has no approved label and therefore no labelled schedule of its own — which is why a question about one has no document to answer it.
What this page does not do
It publishes no conversion procedure, no units-to-milligrams table and no dose figure for any compounded product. It does not name or assess any compounding pharmacy or seller, and it draws no conclusion about the quality of any preparation.
It also cannot say how many of the 1,720-odd reports were dosing errors specifically, because FDA's page gives the mechanisms and the totals separately and does not cross-tabulate them. That is a limit of the source, and the number is reported here as what it is.
Sources and dates
- FDA, "FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss" — content current as of 2026-09-01, read 2026-09-21.
- OZEMPIC prescribing information, dosage forms and strengths, SPL effective 2026-07-30 — read 2026-09-21 via openFDA.
- ZEPBOUND and MOUNJARO prescribing information, dosage forms and strengths, SPL effective 2026-08-28 and 2026-07-29 — read 2026-09-21 via openFDA.
- WEGOVY prescribing information, dosage forms and strengths — read 2026-09-21 via openFDA.
- HUMULIN R prescribing information, dosage forms and strengths, SPL effective 2023-10-31 — read 2026-09-21 via openFDA, for the U-100 definition.