How Many Units Is Your GLP-1 Dose? It Depends on the Vial

There is no universal answer to “how many milligrams is 20 units.” A syringe measures volume; a dose is measured in milligrams of drug; and the number that converts one to the other is the concentration of the vial in front of you, which the pharmacy chose. Twenty units of a 2.5 mg/mL semaglutide vial is 0.5 mg. Twenty units of a 5 mg/mL vial is 1 mg — the same mark on the same syringe, twice the drug.

That is not a technicality. In July 2024 the agency issued a compounding alert on precisely this failure, describing adverse events — some of which sent people to the hospital — where the amount injected came to between five and twenty times what was intended.^[1] This page gives you the arithmetic, the full conversion tables for the concentrations actually used in US compounding, and the one thing you have to read off your own vial before any of it applies.

The arithmetic, in one line

A U-100 insulin syringe — the type shipped with almost every compounded GLP-1 vial — holds 100 units in 1 milliliter. So one unit is 0.01 mL, always, on every U-100 syringe, regardless of what is in it. That gives:

milligrams = (units × concentration in mg/mL) ÷ 100
units = (milligrams × 100) ÷ concentration in mg/mL

The concentration is the only variable, and it is not standardized. FDA's alert says so plainly: prescribers “should be vigilant when prescribing and administering compounded semaglutide, as there may be different concentrations available.”^[1] Two sellers can both advertise “0.25 mg to start” and ship vials that need a different number of units to deliver it.

Compounded semaglutide: units to milligrams

The two concentrations in common US compounding use are 2.5 mg/mL and 5 mg/mL. Find your vial's concentration in the column heading, then read across from the unit mark.

The same table from the other direction

Compounded tirzepatide: units to milligrams

Tirzepatide is usually compounded at 5 mg/mL or 10 mg/mL. Because the approved dose range runs much higher than semaglutide's, the unit numbers get large quickly — which matters, as the second table shows.

Where the numbers stop fitting on one syringe

What actually goes wrong, in FDA's words

FDA's alert is worth reading rather than summarizing, because the failures it describes are ordinary ones. The most common was not exotic:

“In several reports, patients were instructed to use a U-100 (1 milliliter) insulin syringe to draw small doses, such as a 5-unit (0.05 milliliter) dose, from a multiple-dose vial… However, these patients mistakenly administered 50 units instead.”^[1]

— FDA, Alert on dosing errors with compounded injectable semaglutide, 26 July 2024

Five units and fifty units are one decimal place apart and sit at opposite ends of the same barrel. FDA also documents the error running the other way, from the prescriber:

“One provider intended to dose 0.25 milligrams (5 units), but prescribed 25 units instead, leading to a patient receiving five times the intended dose and experiencing severe vomiting. Another provider prescribed 20 units instead of 2 units, affecting three patients who, after receiving 10 times the intended dose, experienced nausea and vomiting.”^[1]

— FDA, Alert on dosing errors with compounded injectable semaglutide, 26 July 2024

One of the reported patients was themselves a health care provider, recalculating their own dose, and still gave themselves ten times what they meant to.^[1] The lesson is not that people are careless. It is that a system where the dose is in milligrams, the syringe is in units, and the conversion factor is a number chosen by a pharmacy you have never spoken to is a system that produces this error.

FDA also notes that in one case the patient could not get a clear answer on dosing from the telemedicine provider who wrote the prescription, and searched online for medical advice instead.^[1] If you are reading this page because that happened to you, the answer is not on this page either — it is on your vial, and from a person you can reach.

Why nobody asks this about a brand pen

Wegovy, Ozempic, Zepbound and Mounjaro are supplied in pens with the dose fixed or dialed in milligrams. There is no volume for the patient to measure, so there is no conversion to get wrong and no concentration to check. That single design difference is the reason this question is asked about compounded vials and essentially never about the approved products.

It is worth weighing when you compare prices. A compounded vial can be a great deal cheaper, and it also moves a measurement step from a factory to your kitchen. Our comparison of compounded and brand semaglutide sets out the rest of that trade, and what is actually in a compounded vial covers what else the label should tell you.

What to check before you draw

1. The concentration on the vial, in mg/mL. Not the total milligrams in the vial — a “10 mg vial” tells you nothing until you know the volume it is dissolved in.
2. That your syringe is U-100. U-40 syringes exist, are used in veterinary insulin, and will deliver 2.5 times what you intend if you read them as U-100.
3. That your prescriber's unit number matches your vial. Run it through the formula above, or the unit converter. If the two disagree, the disagreement is the finding — ask before you inject.
4. That nothing above 100 units is being asked of you without an explanation, since that is more than a full syringe.
5. Who to call. A seller that cannot put you in touch with a person about a dose is a seller to reconsider, whatever the price.

Common versions of this question

If you are still choosing a seller

Concentration is not usually something you can compare before you buy, but two things are: whether a seller names the pharmacy that will fill your vial, and whether it publishes a price before asking for your medical history. Both are in our register for every seller we track. Start at the seller reviews, or the pharmacy directory if you would rather work back from who does the compounding.