How this works
The math
How the arithmetic works
mg from old vial = units ÷ 100 × old strengthmg still needed = dose − mg from old vialunits from new vial = mg still needed ÷ new strength × 100For example, a 10 mg dose with 20 units left at 10 mg/mL is 2 mg from the old vial. The other 8 mg at 20 mg/mL is 40 units. A mark on the syringe means a different amount of medicine for each vial.
What the label shows
- Combining vials
- No label for these medicines describes it.2345 Zepbound’s instructions say: “Do not mix ZEPBOUND with any other medicine.”2
- Different strengths
- FDA says FDA-approved semaglutide injections have standard concentrations, while compounded product concentrations may vary and one compounder may offer several.6
- Older vials
- For compounded multiple-dose vials, FDA says to discard within 28 days after first use, even if medicine remains.1 Zepbound’s multi-dose vial has its own limit: 30 days, or 4 weekly doses.2
- Needles and syringes
- FDA says to always use a new, sterile needle and syringe for each dose, because reuse can contaminate the medication.1
- This page
- It works out two numbers. It can’t tell you whether a leftover amount is usable, whether two vials are the same product, or how to inject them.
- Dates do not establish usability
- An earlier printed expiry or beyond-use date, storage conditions and the vial’s own instructions can shorten a deadline. This arithmetic cannot establish that a vial can be used. Check with your pharmacist first.
- Different vial types
- Do not reuse a single-dose vial. FDA’s 28-day statement concerns compounded multiple-dose vials. Branded multi-dose vials follow their own label, including expiry, cumulative room-temperature exposure and prior dose limits. Use the storage tool and ask your pharmacist; do not infer usability from this page.12
Before you act on this
FDA’s reports include people drawing five to 20 times more than the intended dose from multiple-dose vials, and several who meant to draw 5 units and drew 50.6 Two strengths at once give you two chances to read the wrong number.
Took more than you were prescribed? Call your prescriber or pharmacist now. In the US, Poison Help is 1-800-222-1222. Severe vomiting, severe nausea or signs of low blood sugar need urgent care.
Something went wrong, or a vial looks or acts wrong? Tell your prescriber. In the US, you can also report it to FDA’s MedWatch program.1
Always follow your doctor’s or prescriber’s advice and guidance. Talk to your prescriber before changing your dose.
Read this first
No label describes making one dose from two vials
- Zepbound’s instructions say not to mix it with any other medicine.2 They don’t describe combining two vials of it, and none of the other labels below does either.
- FDA’s dosing-error reports involve multiple-dose vials, and it warns that concentrations can vary, even from one compounder.6 The same number of units from a stronger vial is a bigger dose.
- Ask your pharmacist first: whether to do this at all, and whether it means one syringe or two. This page can’t tell you.
For information only, not medical advice. Verify results with your healthcare provider; Onederland accepts no liability for use of these tools.
- Runs on this device. Nothing you type is sent anywhere.
- Every figure names its source, below.
- Education, not medical advice. Your prescriber comes first.
Sources
Sources
- FDA: concerns with unapproved GLP-1 medicines, October 1, 2026. fda.gov
- Zepbound (tirzepatide) injection, US prescribing information, revised 08/2026. accessdata.fda.gov
- Mounjaro (tirzepatide) injection, US prescribing information, revised 08/2026. accessdata.fda.gov
- Wegovy (semaglutide) injection and tablets, US prescribing information, revised 06/2026. accessdata.fda.gov
- Ozempic (semaglutide) injection, US prescribing information, revised 05/2026. accessdata.fda.gov
- FDA: dosing errors with compounded injectable semaglutide, July 26, 2024. fda.gov
Label sources last checked on October 30, 2026. Onederland summarizes them; your prescriber and the full prescribing information come first.
