Units, mL and mg: reading a compounded tirzepatide vial without a dosing error

Brand tirzepatide pens make dosing a mechanical act: you dial nothing and the pen delivers 0.5 or 0.6 mL that happens to contain the labelled milligrams. A compounded vial hands that arithmetic back to you, and the arithmetic changes every time the concentration does.

The one fact that matters

An insulin syringe is marked in units. On the common U-100 syringe there are 100 unit marks per millilitre, so one unit is 0.01 mL. The marks measure liquid. How much tirzepatide is in that liquid depends entirely on the concentration printed on the vial, in milligrams per millilitre.

So the conversion is always two steps. Volume in mL equals dose in mg divided by concentration in mg/mL. Units equals mL times 100. A 5 mg dose from a 10 mg/mL vial is 0.5 mL, or 50 units. The same 5 mg from a 20 mg/mL vial is 0.25 mL, or 25 units. Same drug, same dose, half the liquid.

Why there is no standard concentration

Even Lilly does not use one. Section 3 of the Zepbound label lists the multi-dose vial and KwikPen contents: 10 mg in 2.4 mL (4.17 mg/mL) for the 2.5 mg dose, 20 mg in 2.4 mL (8.33 mg/mL) for 5 mg, 30 mg in 2.4 mL (12.5 mg/mL) for 7.5 mg, 40 mg in 2.4 mL (16.7 mg/mL) for 10 mg, 50 mg in 2.4 mL (20.8 mg/mL) for 12.5 mg, and 60 mg in 2.4 mL (25 mg/mL) for 15 mg. Each delivers four 0.6 mL doses. Lilly chose to keep the volume constant and let the concentration vary by strength.

Compounding pharmacies make their own choices, and they are not bound to Lilly's. One pharmacy may fill every strength at 10 mg/mL; another at 17 mg/mL; another may change concentration between your first and second fill because it changed vial size. The number on your last vial tells you nothing about this one.

What FDA has seen go wrong

FDA's page on unapproved GLP-1 drugs, current as of September 1, 2026, describes adverse event reports involving compounded semaglutide and tirzepatide in which patients measured and self-administered incorrect doses, and in which health care professionals miscalculated doses. Some patients received amounts above what the approved labels recommend, including escalation faster than the label schedule, with nausea and vomiting as the result. As of May 31, 2026, FDA had received more than 730 adverse event reports for compounded tirzepatide and 990 for compounded semaglutide. FDA notes that pharmacies compounding under section 503A are not required to submit adverse event reports, so the counts understate the problem. The same page states that compounded drugs are not FDA approved and that FDA does not review them for safety, effectiveness or quality before they are marketed.

A checklist before every injection from a vial

1. Read the concentration on this vial. Write it down.
2. Read the dose in milligrams on your prescription. Not the units from a text message, the milligrams.
3. Divide. Convert to units. Compare with the units the pharmacy told you. If they differ, stop and call.
4. Check the syringe is U-100. A U-40 or U-50 syringe has different marks and the units figure is wrong.
5. Check the volume fits the syringe and that you are not measuring under about 5 units, which is hard to do accurately. Ask for a different concentration if so.
6. Count the doses left. A 30 mg vial at 5 mg per dose holds six doses; if you are on your seventh draw something is off.

The converter above does steps 3 and 6 and shows the nearest mark for each common syringe. It never pre-fills a dose or a concentration; both come from your paperwork.

Where this fits

For any drug and any concentration, the dose unit converter on FormBlends Calculators does the same job with a worked example. If you are switching between presentations, pens vs vials vs KwikPen explains what each one contains. If you get compounded tirzepatide through FormBlends, the same rule applies: it is not FDA approved, it is not interchangeable with Zepbound or Mounjaro, and the concentration on the vial you are holding is the only one that counts.