Worked example
Same dose, new vial: why your units changed
Your prescribed dose did not change, but your new vial is a different strength. Here is a worked example of how 7.5 mg of tirzepatide goes from 30 units to about 41.67, and why that matters.
"My new vial looks different. I take 7.5 mg. How many units do I draw now?" It comes up often in online GLP-1 groups, and the quick answers people get are often rounded without saying so.
The example
A made-up but realistic case. Your prescribed dose stays at 7.5 mg of tirzepatide. Your pharmacy changes, and the new vial is a different strength.
| Old vial | New vial | |
|---|---|---|
| Label says | Tirzepatide/Niacinamide 25mg-2mg/mL | Tirzepatide 18 mg/mL |
| Tirzepatide concentration | 25 mg/mL | 18 mg/mL |
| Calculated units for 7.5 mg | (7.5 ÷ 25) × 100 = 30 units | (7.5 ÷ 18) × 100 = about 41.67 units |
Step 1: decode the old label
"25mg-2mg/mL" is not a range. The dash separates two ingredients, listed in the same order as the names: 25 mg/mL of tirzepatide and 2 mg/mL of niacinamide. Only the tirzepatide number is used to calculate your units.
Step 2: see what the old unit count would do
Drawing your usual 30 units from the new 18 mg/mL vial gives (30 ÷ 100) × 18 = 5.4 mg, not 7.5 mg. The reverse is the dangerous direction: moving from 18 mg/mL to 25 mg/mL and keeping 42 units would give 10.5 mg, 40% more than prescribed.
Same mg dose, different concentration, different units. Check the concentration on each new label before your first dose from it.
Step 3: 41.67 units, so the 42 line
7.5 ÷ 18 × 100 is 41.666… units. No syringe has a line at 41.67, so the nearest printed line is 42, on both a syringe with a line at every unit and a 100-unit syringe with lines every 2 units. The difference is about 0.06 mg, less than 1% of the dose.
DoseCompa shows "Draw to the 42-unit line" with the exact 41.67 beside it, so you know it was rounded. It only rounds when the difference is this small: halfway results and small doses still ask you to check with your pharmacist.
Tirzepatide at 18 mg/mL: calculated units
At 18 mg/mL, every common dose works out to a fraction of a unit, so the nearest printed line is shown too:
| Dose | Calculated units | Nearest line, 1-unit lines | Nearest line, 2-unit lines |
|---|---|---|---|
| 2.5 mg | 13.89 | 14 | 14 |
| 5 mg | 27.78 | 28 | 28 |
| 7.5 mg | 41.67 | 42 | 42 |
| 10 mg | 55.56 | 56 | 56 |
| 12.5 mg | 69.44 | 69 | 70 |
| 15 mg | 83.33 | 83 | 84 |
Example doses only, not a dosing schedule. Your prescriber sets your dose. Lines every 2 units are typical of 100-unit (1 mL) syringes. If your pharmacist gave you different instructions, follow them.
What to ask your pharmacist
Have your vial and syringe in hand and ask:
- "What is the tirzepatide concentration of this vial in mg per mL?"
- "My 7.5 mg dose works out to about 41.67 units. Is the 42 line right for me?"
- "Is there a syringe or a vial strength that would make my dose land on a printed line?"
Check your numbers. Enter your vial concentration, your prescribed dose and your syringe, and DoseCompa shows the calculated units and where they fall on your syringe's printed lines.
Open the calculatorQuestions
How many units is 7.5 mg of tirzepatide at 18 mg/mL?
About 41.67 units on a U-100 syringe (7.5 ÷ 18 × 100). The nearest printed line is 42.
What does a label like 25mg-2mg/mL mean?
Usually two ingredients, listed in the same order as the names on the label. In “Tirzepatide/Niacinamide 25mg-2mg/mL”, tirzepatide is 25 mg/mL. If you cannot tell which number is the GLP-1, ask the pharmacy.
Why does one calculator say 42 units and another 41.67?
41.67 is the exact math; 42 is the nearest printed line on the syringe. Both are right. DoseCompa shows both so you can see the rounding is less than 1% of the dose.
Can I keep using my old number of units with a new vial?
Not without recalculating. If the concentration changed, the same units give a different dose. Recalculate from the new label before your first dose.
My dose is the same but my units went up. Did my dose change?
No. A weaker vial needs more liquid for the same mg, so the unit count goes up. A stronger vial needs fewer units.