Units on a U-100 insulin syringe measure volume, not drug. One unit is 0.01 mL, so the milligrams sitting inside that volume depend entirely on the concentration printed on the vial, and the conversion is units = (dose in mg / concentration in mg per mL) x 100.
That is the whole calculation. There is no fixed number of units in a milligram of semaglutide or tirzepatide, and any chart offering one without naming a concentration is wrong for most vials. A 0.25 mg dose works out to 25 units from a 1 mg/mL vial and 2.5 units from a 10 mg/mL vial. Same drug, same milligrams, a ten-fold difference in what the syringe shows.
- A U-100 syringe divides 1 mL into 100 marks, so one unit is 0.01 mL of liquid regardless of what is dissolved in it.
- units = (mg / mg per mL) x 100. Reversed: mg = (units x mg per mL) / 100.
- The same milligram dose can be 2.5 units or 25 units depending on the vial, which is why the concentration has to be read off the label every single time.
- Prefilled pens do not use this arithmetic. Their dose selector is marked in milligrams and the concentration differs between strengths.
- Published case series record ten-fold overdoses traced to confusion between millilitres, units and milligrams (PMID 37392810, PMID 38470137).
- A U-100 syringe divides 1 mL into 100 marks, so one unit is 0.01 mL of liquid regardless of what is dissolved in it.
- units = (mg / mg per mL) x 100. Reversed: mg = (units x mg per mL) / 100.
- The same milligram dose can be 2.5 units or 25 units depending on the vial, which is why the concentration has to be read off the label every single time.
- Prefilled pens do not use this arithmetic. Their dose selector is marked in milligrams and the concentration differs between strengths.
- Published case series record ten-fold overdoses traced to confusion between millilitres, units and milligrams (PMID 37392810, PMID 38470137).
Why a unit is not an amount of drug
The unit marking on an insulin syringe is inherited from insulin, where it does mean something specific. An international unit of insulin is a defined amount of biological activity, and U-100 insulin is formulated so that exactly 100 of those units sit in 1 mL. The syringe barrel is then printed with 100 evenly spaced marks across that millilitre. Reading "20 units" off a U-100 syringe of insulin gives you 20 units of insulin because the formulation was built to make the two line up.
Nothing about that carries over to a GLP-1 drug. Semaglutide and tirzepatide are dosed in milligrams and have no unit at all. When a vial is drawn up with an insulin syringe, the syringe is doing nothing more than measuring 0.01 mL steps. It has no way of knowing what is dissolved in the liquid or how much of it is there.
So "how many units is 1 mg" has no answer on its own. It has an answer only once you add the concentration, and concentration is a property of the specific vial in front of you rather than of the drug.
The formula, both directions
Three lines cover every case:
- Volume needed: dose in mg / concentration in mg per mL = volume in mL
- mg to units: (dose in mg / concentration in mg per mL) x 100 = units on a U-100 syringe
- units to mg: (units x concentration in mg per mL) / 100 = dose in mg
Worked through once: a 0.5 mg dose from a vial labelled 2.5 mg/mL. Divide 0.5 by 2.5 and you get 0.2 mL. Multiply by 100 and you get 20 units. The same 0.5 mg from a vial labelled 5 mg/mL is 0.1 mL, or 10 units.
Note what changed and what did not. The prescribed dose was identical in both cases. The number on the syringe halved, because the second vial was twice as strong.
What each concentration works out to
The table below applies the formula across the concentrations commonly seen on compounded semaglutide vials and the milligram steps used in the semaglutide titration schedule. Every figure is arithmetic, not a recommendation, and it is only correct for a U-100 syringe.
| Vial concentration | 1 unit delivers | 0.25 mg | 0.5 mg | 1 mg | 1.7 mg | 2.4 mg |
|---|---|---|---|---|---|---|
| 1 mg/mL | 0.01 mg | 25 units | 50 units | 100 units | 170 units* | 240 units* |
| 2 mg/mL | 0.02 mg | 12.5 units | 25 units | 50 units | 85 units | 120 units* |
| 2.5 mg/mL | 0.025 mg | 10 units | 20 units | 40 units | 68 units | 96 units |
| 5 mg/mL | 0.05 mg | 5 units | 10 units | 20 units | 34 units | 48 units |
| 10 mg/mL | 0.1 mg | 2.5 units | 5 units | 10 units | 17 units | 24 units |
* More than 100 units, which is more than a standard 1 mL U-100 syringe holds.
Read down any column and the hazard is obvious. The 0.25 mg column runs from 25 units to 2.5 units across a range of concentrations that all exist. Someone who learned "0.25 mg is 25 units" on one vial and applied it to a ten times stronger one would draw ten times the intended dose, and that is not a hypothetical, as the case reports further down show.
U-100, U-40 and why the syringe type matters
U-100 is the standard in human medicine, but it is not the only syringe on the shelf. A U-40 syringe divides 1 mL into 40 marks instead of 100, so one unit on it is 0.025 mL rather than 0.01 mL. U-40 syringes are mostly veterinary, matched to U-40 insulin.
The consequence is a factor of 2.5. Drawing to the "20" mark gives 0.2 mL on a U-100 syringe and 0.5 mL on a U-40 syringe. Any unit figure calculated with the formula above assumes U-100, and the syringe barrel states which it is.
Prefilled pens do not work this way
Almost every "how many units" question comes from a vial and a syringe. Pens sidestep the arithmetic entirely: the dose selector is marked in milligrams, and the device meters the correct volume itself. You never count units on a pen, and you do not draw from one with a syringe.
The table below shows why unit maths would be meaningless across pens anyway. Each strength is formulated at its own concentration.
| Product | Labelled strength | Concentration |
|---|---|---|
| Ozempic pen, 2 mg | Delivers 0.25 mg or 0.5 mg doses | 0.68 mg/mL |
| Ozempic pen, 4 mg | Delivers 1 mg doses | 1.34 mg/mL |
| Ozempic pen, 8 mg | Delivers 2 mg doses | 2.68 mg/mL |
| Wegovy single-dose pen | 0.25 mg in 0.5 mL | 0.5 mg/mL |
| Wegovy single-dose pen | 1 mg in 0.5 mL | 2 mg/mL |
| Wegovy single-dose pen | 2.4 mg in 0.75 mL | 3.2 mg/mL |
| Wegovy HD single-dose pen | 7.2 mg in 0.75 mL | 9.6 mg/mL |
| Mounjaro or Zepbound, 2.5 mg | 2.5 mg in 0.5 mL | 5 mg/mL |
| Mounjaro or Zepbound, 15 mg | 15 mg in 0.5 mL | 30 mg/mL |
Ozempic tops out at a labelled 2 mg dose, which is worth keeping in mind whenever a percentage or a dose figure is attributed to it. The 2.4 mg semaglutide results that circulate widely come from Wegovy trials, and we go through what those trials actually measured separately.
Single-dose vials: 50 units is not a dose
Tirzepatide single-dose vials make the point better than any table. Every Mounjaro and Zepbound single-dose vial holds 0.5 mL, which is 50 units on a U-100 syringe, whichever strength it is. The 2.5 mg vial, the 5 mg vial and the 15 mg vial all measure the same 50 units.
So "50 units" describes six different doses spanning a six-fold range. The number that identifies the dose is the strength printed on the vial, and the whole vial is one dose. The syringe reading is just the volume that gets the vial empty.
This is also the cleanest demonstration that units and milligrams are not two ways of saying the same thing. They are a volume and a mass, and only the concentration connects them.
What the published error reports show
Two peer-reviewed case series describe what happens when the conversion goes wrong.
A 2024 series from the New York City Poison Control Center reported three unintentional semaglutide overdoses at the point of starting treatment. One patient injected 2 mg instead of the intended 0.1 mg. A second injected 2.4 mg instead of 0.25 mg. A third injected 1.7 mg. All three developed nonspecific gastrointestinal symptoms, and none became hypoglycaemic, which the authors attribute to the glucose-dependent mechanism of GLP-1 agonists (PMID 38470137).
A 2023 series reported to a regional poison control centre described three patients given compounded semaglutide by compounding pharmacies and an aesthetic spa. Two self-administered ten-fold dosing errors. One patient reported dosing in millilitres and units rather than milligrams. Another was handed a vial and syringes with no counselling on how to measure a dose. All three had nausea, vomiting and abdominal pain lasting days, and one needed treatment with an antiemetic and intravenous fluids. The authors note that vials of compounded semaglutide lack the safety features of a prefilled manufactured pen and that using syringes not intended for semaglutide adds to the confusion between millilitres, units and milligrams (PMID 37392810).
The FDA issued a compounding risk alert on dosing errors with compounded injectable semaglutide in July 2024 covering the same pattern. Compounded GLP-1 products also sit in a different regulatory position since the semaglutide and tirzepatide shortages were resolved, which has narrowed the circumstances in which compounding is permitted (PMID 42201545).
What to confirm before the first injection of a new vial
This is a checklist of things to read and ask, not a dosing instruction. Dose, product and suitability are decisions for your prescriber.
- The concentration in mg per mL, read off the vial label rather than remembered from a previous one.
- Whether the syringe supplied is U-100 or U-40, which is printed on the barrel.
- The unit figure your prescriber or pharmacist calculates for your prescribed milligram dose with that specific vial and that specific syringe.
- Whether a refill from a different pharmacy carries the same concentration. If it does not, the unit figure changes even though the milligram dose has not.
A mg to units conversion is arithmetic, and it is worth separating from a different question people often ask at the same time: whether a dose of one drug can be translated into an equivalent dose of another. It cannot, and we looked at why cross-drug dose conversion tools have nothing to convert. If what you actually want is what a given dose is likely to do to the scale, the GLP-1 weight loss calculator projects that against published trial curves instead.
Working out what a dose is likely to do, not just how to measure it
Put your starting weight and your drug against the published trial curves and see the projection week by week.
FAQ
How many units is 1 mg?
There is no single answer, because units measure volume and milligrams measure drug. On a U-100 syringe, 1 mg is 100 units from a 1 mg/mL vial, 20 units from a 5 mg/mL vial and 10 units from a 10 mg/mL vial. The concentration printed on the vial is what decides it.
How many units is 0.25 mg of semaglutide?
It depends on the concentration of the vial. Using units = (mg / mg per mL) x 100, a 0.25 mg dose is 25 units from a 1 mg/mL vial, 10 units from a 2.5 mg/mL vial, 5 units from a 5 mg/mL vial and 2.5 units from a 10 mg/mL vial. Semaglutide has no standard concentration across compounded products, so the figure has to be recalculated for each vial.
Is a unit the same as a millilitre?
No. On a U-100 syringe one unit is 0.01 mL, so 100 units is 1 mL and 50 units is 0.5 mL. On a U-40 syringe one unit is 0.025 mL instead, which is why the syringe type has to be checked before any unit figure is used.
How many mg is 50 units?
Multiply 50 by the concentration and divide by 100. At 2.5 mg/mL that is 1.25 mg, at 5 mg/mL it is 2.5 mg, and at 10 mg/mL it is 5 mg. Every Mounjaro and Zepbound single-dose vial happens to hold exactly 0.5 mL, which is 50 units, even though those vials range from 2.5 mg to 15 mg.
Do Ozempic and Wegovy pens use units?
No. Both are dosed in milligrams and the pen meters the volume itself, so there is nothing to convert. Ozempic pens are labelled at 0.68, 1.34 and 2.68 mg/mL for the 2 mg, 4 mg and 8 mg pens, and each Wegovy single-dose pen strength is its own concentration, which is another reason a unit count would not transfer between them.
Why do dosing errors keep happening with this conversion?
Because the same milligram dose maps to wildly different unit counts depending on the vial, and because a vial plus an insulin syringe has none of the built-in limits of a prefilled pen. Published case series record ten-fold errors and injections of 2 mg where 0.1 mg was intended, with confusion between millilitres, units and milligrams named as a contributing factor (PMID 37392810, PMID 38470137).
Sources
- Wiener BG, Gnirke M, Vassallo S, Smith SW, Su MK. Challenges with glucagon-like peptide-1 (GLP-1) agonist initiation: a case series of semaglutide overdose administration errors. Clin Toxicol (Phila). 2024;62(2):131-133. PMID: 38470137
- Lambson JE, Flegal SC, Johnson AR. Administration errors of compounded semaglutide reported to a poison control center: case series. J Am Pharm Assoc (2003). 2023;63(5):1643-1645. PMID: 37392810
- Trainer N. The "microdosing" dilemma: balancing patient anecdotes with clinical safety amid GLP-1 compounding restrictions. J Am Assoc Nurse Pract. 2026. PMID: 42201545
- US Food and Drug Administration. FDA alerts health care providers, compounders and patients of dosing errors associated with compounded injectable semaglutide products. Compounding risk alert, July 2024.
- Ozempic (semaglutide) injection, US prescribing information, Novo Nordisk. Pen concentrations 0.68, 1.34 and 2.68 mg/mL; maximum labelled dose 2 mg.
- Wegovy (semaglutide) injection, US prescribing information, Novo Nordisk. Single-dose pen strengths 0.25, 0.5, 1, 1.7, 2.4 and 7.2 mg.
- Mounjaro and Zepbound (tirzepatide) injection, US prescribing information, Eli Lilly. Each single-dose pen or vial contains 2.5 to 15 mg per 0.5 mL.
This article is for information only and is not medical advice. Dosing, suitability and any change to treatment are decisions for your prescriber.