Safety
Units or milligrams? The dosing mistake behind 1,700 FDA reports
A prescriber meant 0.25 mg, which is 5 units. They wrote 25 units. That is a ten-fold overdose, and the FDA has logged over 1,700 adverse event reports on compounded GLP-1s.
The short version
A dose is in milligrams. A syringe is marked in units. They are not the same kind of thing, and the number that converts one to the other is different for every vial.
On a standard U-100 insulin syringe, 100 units = 1 mL. So a unit is a hundredth of a millilitre — a measure of volume. How many milligrams of drug sit in that volume depends on the concentration the pharmacy made.
The arithmetic, once
`` units = (milligrams ÷ concentration in mg per mL) × 100 ``
The same 2.5 mg prescription, in three vials a pharmacy might send:
| Vial concentration | 2.5 mg is | On a U-100 syringe |
|---|---|---|
| 5 mg/mL | 0.5 mL | 50 units |
| 10 mg/mL | 0.25 mL | 25 units |
| 20 mg/mL | 0.125 mL | 12.5 units |
Same prescription. Four times the difference between the top and bottom row. If you switch pharmacies, or your pharmacy changes its formulation between refills, the number of units changes even though your dose has not.
What actually went wrong
The FDA's alert describes the failure plainly: a prescriber intended 0.25 mg, which was 5 units, and wrote 25 units instead. Ten times the dose.
Others came from converting milligrams to units or millilitres incorrectly, with patients receiving five to ten times what was intended.
The reported consequences: nausea, vomiting, abdominal pain, fainting, headache, migraine, dehydration, acute pancreatitis and gallstones.
By 31 May 2026 the FDA had logged 990 reports involving compounded semaglutide and more than 730 involving compounded tirzepatide. The agency notes these are likely undercounts, because state-licensed pharmacies are not required to report adverse events at all.
Why the branded products do not have this problem
Wegovy and Zepbound are supplied as pens and single-dose vials that deliver a fixed dose in milligrams. You do not draw anything, you do not read a scale, and there is no concentration to know. The 2.5 mg starting dose of Zepbound is one device delivering 2.5 mg.
The error described above is not possible with them. That is not a small distinction — it is the entire difference between a dose you administer and a dose you calculate.
If you already have a vial and a syringe
Where we stand
This category of error is why, in MED Miami's program, the pharmacy label and your clinician's instructions state the dose for your vial, and a dose question always goes to a licensed clinician, never an automated reply. If anything on your label is unclear, ask before you draw.
Read next
Where this came from
- FDA, "FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss" — fda.gov
- FDA alert on dosing errors with compounded injectable semaglutide, as reported 2026 — drugdiscoverytrends.com
- FDA, Highlights of Prescribing Information: ZEPBOUND (tirzepatide) injection, 2025 revision — accessdata.fda.gov