A patient is told to draw "5 units" of compounded semaglutide from a vial. They pick up the 1 mL insulin syringe the pharmacy sent, find a "5" printed near the halfway mark, and draw 50 units. That is ten times the intended dose. The FDA describes this exact scenario in its adverse event reports, and it happens because insulin syringes use a unit system most people have never had to read. This article teaches you how to read one. It does not tell you what dose to take. That number comes from your prescriber, and you should confirm the math below with your dispensing pharmacist before you inject anything.
The one fact that unlocks the syringe
Standard insulin syringes are "U-100" syringes. The scale printed on the barrel divides 1 milliliter (mL) into 100 "units":
- 100 units = 1 mL
- 1 unit = 0.01 mL
On the syringe itself, a "unit" measures volume, not medication. Ten units is 0.1 mL of liquid. How much drug is in those 10 units depends on the concentration of the liquid you're drawing from.
Concentration links mg to units
Your prescription is written in milligrams (mg). Your syringe reads in units of volume. The number that connects them is the concentration printed on your vial's label, in mg per mL. The conversion is one line of arithmetic:
units = (dose in mg ÷ concentration in mg/mL) × 100
Divide the dose by the concentration to get milliliters, then multiply by 100, because each mL is 100 units.
Why the same dose can be a different number of units
This is the main way compounded vial-and-syringe dosing goes wrong. Compounding pharmacies use different concentrations, so a unit count copied from someone else, or from your own previous vial, can be flatly wrong. Here is the same prescribed dose at two concentrations.
Example 1: 0.25 mg dose, vial labeled 2.5 mg/mL. 0.25 ÷ 2.5 = 0.1 mL. Then 0.1 × 100 = 10 units.
Example 2: the same 0.25 mg dose, vial labeled 5 mg/mL. 0.25 ÷ 5 = 0.05 mL. Then 0.05 × 100 = 5 units.
Same prescription, half the units, because the second vial is twice as concentrated. If you drew 10 units from that second vial, you'd take double your dose.
Example 3: 1 mg dose, vial labeled 2.5 mg/mL. 1 ÷ 2.5 = 0.4 mL. Then 0.4 × 100 = 40 units.
So "how many units do I draw?" has no universal answer, and forum posts and screenshots with specific unit counts are dangerous to copy. Our guide on GLP-1 microdosing makes the same warning at length. The only concentration that matters is the one on your current vial's label, and it can change between refills.
Syringe sizes and their markings
U-100 insulin syringes come in three common barrel sizes, and the markings differ:
- 0.3 mL holds 30 units. It's usually marked in 1-unit steps, and some models add half-unit marks. It's the easiest to read for small doses.
- 0.5 mL holds 50 units, usually marked in 1-unit steps.
- 1.0 mL holds 100 units and is often marked only in 2-unit steps, with numbers at 10, 20, 30 and so on.
Before you read any barrel, check that the syringe really is a U-100 syringe. Syringes marked for other concentrations exist. U-40 syringes, for example, are common for veterinary insulin, and one case report describes a patient who drew his U-100 insulin with U-40 syringes and had repeated low blood sugar episodes (Barrera and Murvelashvili, Case Reports in Endocrinology, 2019, DOI (external link)). On those syringes the unit lines stand for a different volume, and the arithmetic above doesn't apply. On a 0.3 mL syringe with half-unit markings, count carefully: the short lines between whole units are half-units.
Small doses on a big syringe are where misreads happen. On a 1 mL barrel, a 5-unit dose sits just above the needle, while the printed "50" sits mid-barrel and looks like a reasonable target. The FDA recommends that providers supply a syringe size that fits the intended dose. If your dose is 5 units, ask whether a 0.3 mL syringe makes sense.
What the FDA has documented
In a July 2024 alert, the FDA reported adverse events, some requiring hospitalization, from dosing errors with compounded injectable semaglutide. Most reports described patients drawing more than the prescribed dose from a multiple-dose vial and taking five to 20 times the intended dose. Confusion between milliliters, milligrams, and "units" contributed. The 5-units-versus-50-units mistake above is the FDA's own example. Providers made errors too: several reports describe clinicians miscalculating mg-to-unit conversions, producing five to 10 times the intended dose. Reported adverse events included nausea, vomiting, abdominal pain, fainting, headache, dehydration, acute pancreatitis, and gallstones. A separate poison-control case series in the Journal of the American Pharmacists Association found the same pattern, including 10-fold self-administration errors (Lambson et al., 2023, DOI (external link)).
Compounded GLP-1 vials are also less common than they were when that alert came out. The FDA declared the tirzepatide shortage resolved on December 19, 2024, and the semaglutide shortage resolved on February 21, 2025, which ended the period when compounders could make copies of these drugs at scale. See the compounding debate for what that means for access as of September 2026. If you do have a compounded vial, the arithmetic here still applies.
Checklist for your pharmacist
Before your first injection from any new vial, including refills, confirm with the dispensing pharmacist:
- The concentration on this vial's label, in mg/mL, read aloud from the vial in your hand.
- Your prescribed dose in mg, and the resulting unit count and mL volume. Ask the pharmacist to state it; don't just offer your own answer for approval.
- That your syringe size fits the dose, and which line you'll fill to, ideally shown to you.
- Whether the concentration changed since your last fill, and the new unit count if it did.
- What to do if you draw the wrong amount: who to call, and which symptoms need medical attention.
Reading a U-100 syringe comes down to one fact (100 units = 1 mL) and one line of division. The risk comes from assuming a unit count instead of working it out from your own vial's labeled concentration at every fill.
Study findings above are attributed to their authors and were located via PubMed.
This article is arithmetic education, not medical advice, and it does not prescribe or endorse any dose. Your dose comes from your prescriber; your unit count should be confirmed by your dispensing pharmacist.