Quick answer: Zepbound vials are already liquid and aren't reconstituted; compounded powder follows the pharmacy's directions. Zepbound's FDA label describes every presentation, vials included, as a "clear, colorless to slightly yellow solution," and Lilly's vial instructions go straight from checking the vial to drawing up the dose, with no mixing step; they also say, "Do not mix ZEPBOUND with any other medicine." Mounjaro, and most compounded tirzepatide from pharmacies, also come as a ready-to-use liquid with the concentration on the label. Only if your pharmacy or prescriber supplied a freeze-dried powder is there anything to mix, and then you use the diluent and volume they specify.
The math is then standard: concentration (mg/mL) = mg in the vial ÷ mL added; mL to draw = prescribed mg ÷ concentration; U-100 units = mL × 100.
The rest of this guide covers how to tell which kind you have, how to read each type of label, worked examples and how long a vial lasts.
Educational only, not medical advice. This guide explains labels and arithmetic. It doesn't recommend a dose, a dilution or any product, and it doesn't cover where to get tirzepatide. Your dose and preparation instructions come from your prescriber and pharmacy.
Does your tirzepatide need reconstituting?
| What you have | Needs mixing? | Where the concentration comes from |
|---|---|---|
| Zepbound or Mounjaro single-dose pen | No | Set by the pen; you don't measure anything |
| Zepbound single-dose vial | No | Label, e.g. "5 mg/0.5 mL"; the whole vial is one dose |
| Zepbound multi-dose vial or KwikPen | No | Label, e.g. "10 mg/2.4 mL (4.17 mg/mL)" |
| Compounded tirzepatide solution | No | Pharmacy label, usually in mg/mL |
| Tirzepatide supplied as a powder | Yes | You calculate it: vial mg ÷ mL of diluent added |
If your vial contains liquid, don't add anything to it. Adding diluent to a solution changes its concentration and every draw after that. For Zepbound vials, follow Lilly's single-dose vial instructions for use, which you draw from directly.
Brand-name vials: reading the label
Zepbound's US prescribing information lists two vial types:
- Single-dose vials, each containing one dose in 0.5 mL. The label shows strengths such as "5 mg/0.5 mL." Single-dose vials are meant for one injection.
- Multi-dose vials (and single-patient-use KwikPens) containing 2.4 mL. In the label's words: "Multi-dose vial or single-patient-use KwikPen®: 10 mg/2.4 mL (4.17 mg/mL) for four 2.5 mg/0.6 mL doses," with higher strengths following the same four-dose pattern.
For vials, the label tells prescribers to "Instruct patients using ZEPBOUND vials to use a syringe appropriate for dose administration (e.g., a 1 mL syringe capable of measuring a 0.5 mL or 0.6 mL dose) and always use a new syringe and needle for each injection."
Math only: 0.5 mL is 50 units on a U-100 syringe and 0.6 mL is 60 units, which is why the label mentions a 1 mL syringe. Follow the Instructions for Use that came with your vial.
Compounded tirzepatide: reading the label
Compounded labels vary. Look for:
- Concentration in mg/mL, such as "10 mg/mL," or a total over a volume, such as "20 mg/2 mL" (also 10 mg/mL).
- Total mg and total volume, so you know how many doses the vial holds.
- A beyond-use date.
- Instructions in mg, mL or units. If they're in units only, convert them to mg (units ÷ 100 × concentration) and check that the result matches what your prescriber told you in mg.
Compounded concentrations can change between orders, even from the same pharmacy. The American Diabetes Association has noted that compounded products may be made in varying concentrations, and the FDA has linked "unit" instructions and varying concentrations to dosing errors. Every time a new vial arrives, check its concentration against the last one. Our guide on recalculating units for a new vial explains how.
If your tirzepatide is a powder
If the pharmacy or prescriber supplied a powder, follow their reconstitution instructions exactly: which diluent, how much, and how to mix it. The general technique (swab, add the diluent slowly down the inside wall, swirl gently, don't shake, check that it's clear) is in our peptide reconstitution guide. Then:
- Concentration = vial mg ÷ mL of diluent added
- mL to draw = prescribed mg ÷ concentration
- U-100 units = mL × 100
The amount of diluent changes the concentration, not the mg in the vial. Our post on how much BAC water to add shows what different volumes do to the units per mg.
Reconstitution calculator
Enter the numbers from your own vial, and pick the syringe in your hand. Check the U- number and size printed on the syringe box.
Concentration
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Draw volume
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Syringe units
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U-100 1 mL, holds 100
Draws in the vial
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Math only, not dosing advice. Follow your prescriber or label. Real vials give slightly less than the full-draw count: a little liquid stays in the vial and needle. Your last setup is saved in this browser only; Clear removes it.
Save your vial in Caliper and it remembers the concentration and use-by date you entered, alongside your dose log. The calculator above is a browser tool and is not part of the app.
Save my vial in CaliperWorked examples (math only)
The numbers below illustrate the arithmetic only. None is a dose suggestion; your prescribed amount comes from your prescriber.
Example 1: Compounded solution labeled 10 mg/mL
A hypothetical amount of 2.5 mg:
- 2.5 ÷ 10 = 0.25 mL
- 0.25 × 100 = 25 units
Example 2: Compounded solution labeled "20 mg/2 mL"
- 20 ÷ 2 = 10 mg/mL, the same as Example 1. The math and the units are identical even though the label looks different.
Example 3: A different concentration from the next order
The next vial is labeled 8 mg/mL. The same 2.5 mg:
- 2.5 ÷ 8 = 0.3125 mL ≈ 31.3 units
That falls between ticks on most barrels. Ask the pharmacy how they expect it to be measured rather than rounding on your own.
Example 4: Powder vial with diluent added
A hypothetical 10 mg powder vial, mixed per instructions with 2 mL:
- Concentration: 10 ÷ 2 = 5 mg/mL
- 2.5 mg ÷ 5 = 0.5 mL = 50 units
The same vial mixed with 1 mL would be 10 mg/mL, and 2.5 mg would be 25 units.
Example 5: Units back to mg (checking an instruction)
Instructions say 15 units from a 10 mg/mL vial:
- 15 ÷ 100 = 0.15 mL × 10 = 1.5 mg
If that doesn't match your prescription in mg, call the pharmacy before drawing.
Example 6: How many doses in the vial?
A compounded vial holding 2 mL at 10 mg/mL contains 20 mg in total. Math only: at 2.5 mg per dose that's 8 doses, at 5 mg it's 4. In practice a little is lost to syringe dead space with each draw, and the vial's use-by date may arrive before it's empty.
How long does a tirzepatide vial last?
- Brand single-dose pens and vials: refrigerate at 36–46°F (2–8°C). The label allows up to 21 days unrefrigerated at up to 86°F (30°C); don't put it back in the fridge after that.
- Zepbound multi-dose vial or KwikPen: per its label, throw away an opened one "after a total of 30 days at room temperature, 30 days after first use, or after taking 4 weekly doses, even if there is medicine left in it."
- Compounded multi-dose vials: use them by the pharmacy's beyond-use date. The FDA recommends discarding compounded GLP-1 multi-dose vials within 28 days of first use, even if medicine remains and even if the instructions say longer.
- Never freeze tirzepatide, and don't use it if it has frozen or looks cloudy, discolored or has particles. The label describes it as clear and colorless to slightly yellow.
More detail is in our storage guide.
Common mistakes
- Adding water to a liquid vial because a video said to "reconstitute" it.
- Reusing an old unit count with a new concentration.
- Using a syringe that's too small for the volume, or the wrong scale (U-40 instead of U-100).
- Assuming "20 mg" is the concentration when it's the total in the vial.
- Keeping a multi-dose vial past 28 days to use up the last of it.
- Reusing single-dose vials.
Tracking it in Caliper
Caliper saves each tirzepatide vial with its concentration (or the powder's mg and the diluent you added), its opened or reconstitution date and its use-by date. Dose logs record the amount you entered and where you injected it, and reminders keep your weekly shot on schedule. Caliper never suggests or calculates a dose; it records what you and your provider decide.
FAQ
Does tirzepatide need to be reconstituted?
Usually not. Zepbound vials are already liquid and aren't reconstituted, and Mounjaro and most compounded tirzepatide also come as ready-to-use solutions. Only a vial supplied as a freeze-dried powder needs reconstituting, using the diluent and volume your pharmacy specifies.
Do I add BAC water to a Zepbound vial?
No. Zepbound vials are already a solution. Lilly's vial instructions say to draw the dose directly and "Do not mix ZEPBOUND with any other medicine."
How much BAC water do I add to tirzepatide?
Only add diluent if your tirzepatide is a powder, and use the volume your pharmacy or prescriber specifies. The math: concentration = vial mg ÷ mL added. Never add water to a vial that's already liquid.
How many units is 2.5 mg of tirzepatide?
It depends on the concentration. At 10 mg/mL it's 0.25 mL, or 25 units on a U-100 syringe; at 5 mg/mL it's 50 units. Math only; your dose comes from your prescriber.
How many units is a Zepbound single-dose vial?
Each single-dose vial contains 0.5 mL, which is 50 units on a U-100 syringe. Follow the vial's Instructions for Use.
How long does compounded tirzepatide last once opened?
Use it by the beyond-use date on the label. The FDA recommends discarding compounded GLP-1 multi-dose vials within 28 days of first use, even if medicine remains.
My new compounded tirzepatide vial has a different concentration. What do I do?
Recalculate from your prescribed mg: mL = mg ÷ the new concentration, then units = mL × 100. Confirm with your pharmacy before your first dose from the new vial.
Can I use a vial from one pharmacy with instructions from another?
No. Units only make sense for the vial they were calculated for. Use the concentration on the vial in front of you.
The takeaway
Check whether you even need to mix it. Most tirzepatide is already a solution, so the job is reading its concentration correctly and converting your prescribed mg to mL and units. If you do have a powder, use your pharmacy's diluent and volume, write the concentration on the vial, and recalculate every time a new vial arrives.
Sources
- Zepbound (tirzepatide) US prescribing information, Eli Lilly (revised 08/2026), also on DailyMed
- Zepbound (tirzepatide) single-dose vial instructions for use, Eli Lilly (revised April 2026)
- Mounjaro (tirzepatide) medication guide, DailyMed
- FDA, FDA's Concerns with Unapproved GLP-1 Drugs Used for Weight Loss
- American Diabetes Association, Compounded GLP-1 and dual GIP/GLP-1 receptor agonists: a statement
Make the math (and the record-keeping) automatic
Caliper is a private, local-first tracker for GLP-1 and peptide protocols. You record the amount you took and it logs each dose with one tap, reminds you when the next one is due, tracks vials with reconstitution and use-by dates, and keeps injection sites, side effects and weight trends on one timeline. Your health data stays on your device, and Caliper never recommends or calculates a dose; it organizes what you and your provider decide.
Start tracking your vials and doses with Caliper →
For adults 18+.
Medical disclaimer: This article is for general educational purposes only and is not medical advice. It does not recommend any dose, dilution, compound, product, supplier or protocol. Always follow your prescriber's instructions and the product label, and talk to a pharmacist or healthcare provider about any question regarding your medication, its preparation, storage, measurement or administration.