Quick answer: Switching from a GLP-1 pen to a vial does not change your dose - the pen and vial are the same molecule, so the same mg stays the same mg. What changes is that you now draw the dose yourself: divide your vial size (mg) by the bacteriostatic water (mL) to get the concentration, then units = (your dose ÷ concentration) × 100 on a U-100 syringe. Keep your weekly schedule.
Switching from a pen to a vial, explained
A branded GLP-1 pen and a reconstituted vial of the same generic are the same molecule. A Zepbound or Mounjaro pen delivers tirzepatide; a Wegovy or Ozempic pen delivers semaglutide. If you move to a vial of that same generic and dose it to the same number of milligrams, your dose does not change - you are taking the identical amount of the identical drug. The only thing that changes is that the pen used to measure the dose for you, and now you measure it yourself on an insulin syringe.
That is the whole job of this tool: it turns your weekly pen dose in mg into the number of units to draw from your vial. There is no single conversion, because the same mg dose is a different number of units depending on how much bacteriostatic water you used to reconstitute. The calculator above does the math for your exact vial.
Because nothing about the molecule or the dose changes, there is no washout period when you switch. Keep your weekly cadence: take your first vial dose on your normal injection day, about 7 days after your last pen dose.
Pen dose strengths by brand
| Pen | Molecule | Weekly dose strengths |
|---|---|---|
| Zepbound | Tirzepatide | 2.5, 5, 7.5, 10, 12.5, 15 mg |
| Mounjaro | Tirzepatide | 2.5, 5, 7.5, 10, 12.5, 15 mg |
| Wegovy | Semaglutide | 0.25, 0.5, 1, 1.7, 2.4, 7.2 mg |
| Ozempic | Semaglutide | 0.25, 0.5, 1, 2 mg |
Any result over 100 units exceeds a single U-100 syringe. Use a higher concentration (less water) or split the dose into two draws.
Worked example
Say you are on a 5 mg Zepbound pen each week and switching to a 30 mg Tirzepatide vial reconstituted with 3 mL of bacteriostatic water. That gives 10.0 mg/mL. Your dose is still 5 mg, so: (5 ÷ 10.0) × 100 = 50.0 units (0.5 mL) on a U-100 syringe. Draw to that mark, on your normal injection day.
Frequently asked questions
Is a GLP-1 pen the same dose as a vial?
Yes, for the same molecule. A 10 mg Zepbound pen and a tirzepatide vial dosed to 10 mg both deliver 10 mg of tirzepatide. Switching from a pen to a vial changes how you measure and draw the dose, not the dose itself.
How many insulin units do I draw from my vial?
It depends entirely on how you reconstituted the vial. Divide the vial size (mg) by the bacteriostatic water (mL) to get the concentration, then units = (dose ÷ concentration) × 100. Enter your exact numbers above for the precise answer.
When do I take my first vial dose after my last pen?
Keep your weekly cadence. There is no washout when the molecule and dose are unchanged: take your first vial dose on your normal injection day, about 7 days after your last pen dose. Confirm timing with your prescriber.
Safety: Branded GLP-1 pens (Zepbound, Mounjaro, Wegovy, Ozempic) are FDA-approved prescription medications. A self-reconstituted vial is not an FDA-approved finished medication, even when it contains the same molecule. This tool is educational only and does not recommend switching. Always double-check your own math and follow a licensed provider's instructions before injecting.