Units are not milligrams
A U-100 syringe mark is a volume. The milligrams in that volume come from the vial in your hand. The FDA's compounding alerts are mostly this mix-up.
Key takeaways
- Units on an insulin syringe are not milligrams. They are a volume mark. The milligrams you draw depend on the concentration printed on that specific vial.
- The FDA has logged compounding errors in which people injected five to twenty times the intended dose of semaglutide because mg and units were treated as interchangeable.
- A calculator is only as honest as the concentration you type in. If the vial says 2.5 mg/0.5 mL, that is a different draw from 5 mg/mL for the same milligram dose.
- This page does not recommend a dose. It explains the conversion so a number a prescriber already gave you can be drawn without mixing the two units.
A unit is a volume. A milligram is a mass
U-100 insulin syringes are marked in units. On that syringe, 100 units is 1 mL. 10 units is 0.1 mL. That conversion never changes. What does change is how many milligrams of drug sit in that 0.1 mL, because that is a property of the vial in your hand, not of the syringe.
Treating "10 units" as "10 mg" is how people overshoot. The two numbers only match if the pharmacy compounded the vial at 100 mg/mL, which most compounded GLP-1 vials are not.
The label on this vial, not the last one
Compounded products vary by pharmacy and by batch. The FDA's public concern with unapproved GLP-1s includes identity, purity and concentration that are not the same as the licensed pens. The practical rule is boring: read the concentration on the vial you are about to draw from, every time, even if the last vial from the same pharmacy looked identical.
If the print says 10 mg/mL, then 10 units (0.1 mL) is 1 mg. If it says 5 mg/mL, 10 units is 0.5 mg. Same syringe mark. Different drug. The tirzepatide units calculator and the semaglutide one do that arithmetic. They do not invent a concentration.
What the FDA logged
In 2024 the FDA alerted clinicians, compounders and patients to dosing errors with compounded injectable semaglutide. People had injected five to twenty times the intended dose. The agency's concern was not a mystery molecule. It was people (and in some reports, pharmacies) mixing up milligrams, millilitres and syringe units on products that do not come in a licensed, labelled pen.
That is also why a page that only says "10 units of tirzepatide" without the vial concentration is not an answer. It is half an instruction.
Use the calculator without guessing
Enter the concentration from the vial, then the milligram dose you were already given. The output is a unit mark on a U-100 syringe. If you do not have a concentration, you do not have a conversion. Call the pharmacy and read the vial together rather than assuming last month's number.
Approved pens (Ozempic, Wegovy, Mounjaro, Zepbound) are not drawn in insulin units from a multi-dose vial. Those devices have their own click or milligram markings. This conversion is for compounded vials. Mixing the two systems is another way the same error happens.
Common questions
- How many units is 2.5 mg of tirzepatide?
- It depends on the vial. At 5 mg/mL, 2.5 mg is 0.5 mL, which is 50 units on a U-100 syringe. At 10 mg/mL, 2.5 mg is 0.25 mL, 25 units. There is no single answer without the concentration.
- Can I use a U-40 syringe?
- The calculators on this site assume U-100 (100 units = 1 mL). A U-40 syringe uses a different scale. Do not reuse a unit number from a U-100 page on a U-40 barrel.
- Is this the dose I should take?
- No. The dose is the milligram number a prescriber already gave you. This page only converts that number into a syringe mark for a stated concentration.
Sources
Every figure on this page traces to one of these. Regulatory documents and peer-reviewed primary research only — see our editorial policy for what we accept as a source.
- FDA Alerts Health Care Providers, Compounders and Patients of Dosing Errors Associated with Compounded Injectable Semaglutide Products. FDA MedWatch, 2024. View source
- FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss. U.S. Food and Drug Administration. View source
- Compounded GLP-1s: why doctors worry and the FDA is cracking down. Stanford Medicine, 2026. View source
Do the conversion, then log the vial
The useful record is concentration, milligrams, units drawn, and the date. GLPme keeps dose history in one timeline. Core tracking is free.
