Drug facts

Units, milligrams and milliliters

Reviewed August 2026

If you have a vial in front of you and want the mark on the syringe, work it out here — it needs the concentration printed on your own label, because a compounded vial’s strength is set by the pharmacy that made it.

Quick answer

A unit is a mark on a syringe, not an amount of medicine. On a standard U-100 insulin syringe, 100 units is one milliliter of liquid. How many milligrams of tirzepatide that liquid contains depends entirely on the concentration of your particular vial — so the same milligram dose can be a different number of units from a different pharmacy.

Why there are two different numbers at all1

Doses of tirzepatide are written in milligrams — 2.5 mg, 5 mg, 7.5 mg. That is a quantity of drug. But a syringe cannot measure drug; it measures liquid. Insulin syringes, which are what compounded vials are usually drawn with, are marked in units, and on the standard U-100 syringe those marks divide one milliliter into a hundred parts.

So “units” answers how much liquid, and “milligrams” answers how much drug. The bridge between them is the concentration of your vial, written as milligrams per milliliter.

Why the same dose is a different number of units at different pharmacies2

Compounded vials are not standardized. One pharmacy may supply a concentration that puts your dose near the middle of the syringe; another may supply a more concentrated vial where the identical milligram dose is a much smaller mark. Neither is wrong, but they are not interchangeable, and a number you remember from a previous vial can be badly wrong on a new one.

This is the single most consequential difference between a compounded vial and an approved pen, and it is why the instruction that arrives with your vial matters more than any general chart.

On the label — Mounjaro / Zepbound

Supplied in single-dose pens and vials at a fixed strength. The dose is the device: there is nothing to measure and no concentration to check.

In a compounded vial

Concentration is chosen by the compounding pharmacy and printed on the vial. Two vials of the same milligram strength can hold different concentrations, so the mark on the syringe is not transferable between them.

What to check on your own vial3

Your label should tell you the concentration in milligrams per milliliter, the total amount in the vial, and a beyond-use date. Those three things together are what make the instructions from your prescriber meaningful. If any of them is missing from your label, that is worth raising with the pharmacy before you use it.

If you want to understand the beyond-use date, see storage and beyond-use dating.

The questions this page cannot answer

How many units to draw. Whether to change your dose. What to do about a missed dose. Whether a vial that looks different from the last one is still correct. Every one of those depends on your vial and your history, and belongs to the prescriber and pharmacy who supplied them.

Sources

  1. U-100 syringe convention; FDA prescribing information for ZEPBOUND, section 3, revision 4/2026 DailyMed
  2. U-100 syringe convention; concentrations as published by compounding sellers
  3. General labeling practice for compounded preparations