Quick answer: only a semaglutide vial that holds freeze-dried powder is reconstituted. Wegovy and Ozempic come as ready-to-use pens or prefilled syringes, and a vial that already holds liquid should never have anything added to it. For a powder, you add the diluent and volume your prescriber, pharmacy or label specifies, then work out the draw with three steps: concentration (mg/mL) = mg in the vial ÷ mL added; mL to draw = prescribed mg ÷ concentration; U-100 units = mL × 100. For example, a 5 mg vial with 2 mL added is 2.5 mg/mL, so 0.5 mg is 0.2 mL, or 20 units.
The steps, a units table for common vial sizes, worked examples and the mistakes to avoid are below.
Educational only, not medical advice. This guide explains handling steps and arithmetic. Every number in it is an illustration of the math, not a dose, a dilution or a suggestion. Your amount and your diluent volume come from your prescriber, pharmacy instructions and the product label.
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 CaliperDoes semaglutide need to be reconstituted?
| What you have | Mix anything in? | Where the concentration comes from |
|---|---|---|
| Wegovy or Ozempic pen or prefilled syringe | No | Set by the pen; follow its Instructions for Use |
| A vial of liquid (clear solution, concentration on the label) | No | The label, e.g. "5 mg/mL" or "10 mg/2 mL" |
| A vial of freeze-dried powder | Yes | You calculate it: vial mg ÷ mL of diluent added |
Wegovy and Ozempic are supplied as solutions in pens or prefilled syringes, so there is nothing to reconstitute; follow the Instructions for Use that came with the pen. If your vial contains liquid, don't add water. Adding diluent to a solution weakens its concentration, and every unit count based on the label stops being true.
If you're not sure which kind of vial you have, ask your pharmacist before you open it. A powder looks like a small white or off-white cake or film at the bottom of the vial; a solution is a clear liquid.
How to reconstitute semaglutide step by step
This is the general sequence most product instructions follow. Your prescriber's, pharmacist's or product's instructions come first, including which diluent to use and how much.
- Read the label and your instructions. Note the amount in the vial (mg), the diluent you were told to use, and the volume of it.
- Gather clean supplies: the vial, the specified diluent (often bacteriostatic water), a sterile syringe and needle for each puncture, alcohol swabs and a sharps container.
- Wash your hands and swab the stoppers of both vials. Let the alcohol air-dry.
- Draw up exactly the volume of diluent your instructions give. Check the mL against the syringe marks: on a U-100 syringe, 100 units is 1 mL.
- Add the diluent slowly down the inside wall of the vial, not straight onto the powder.
- Swirl or roll the vial gently until it dissolves. Don't shake it. Some powders take a few minutes.
- Look at it. The solution should be clear. If it's cloudy, has particles or looks discolored, don't use it, and see why a peptide can look cloudy after reconstitution.
- Label the vial with the date you mixed it, the volume you added and the concentration (mg ÷ mL).
- Store it as the label directs, usually in the refrigerator, and note the use-by date. See our storage guide.
The general technique is the same for any powder; our peptide reconstitution guide covers it in more depth.
The units math: mg to mL to syringe units
Three steps turn the vial label and your prescribed mg into a number on the syringe:
- Concentration (mg/mL) = mg in the vial ÷ mL of diluent added
- mL to draw = prescribed mg ÷ concentration
- U-100 units = mL × 100 (1 unit is 0.01 mL)
Check it backwards: units ÷ 100 × concentration = mg. If that doesn't give back your prescribed mg, something is off.
A syringe unit is a volume, not an amount of drug. How much semaglutide sits in 10 units depends entirely on the concentration, which is why a unit count is only meaningful for the vial it was calculated from. Our guide on why units don't equal mg goes deeper.
Units by vial size and diluent volume
Math only. The amounts are round numbers for illustration, not suggestions. Results such as 7.5 units fall between marks on most syringes; see how to read insulin syringe tick marks.
| Vial | Diluent added | Concentration | 0.25 mg | 0.5 mg | 1 mg |
|---|---|---|---|---|---|
| 2 mg | 1 mL | 2 mg/mL | 12.5 units | 25 units | 50 units |
| 5 mg | 1 mL | 5 mg/mL | 5 units | 10 units | 20 units |
| 5 mg | 2 mL | 2.5 mg/mL | 10 units | 20 units | 40 units |
| 5 mg | 3 mL | 1.67 mg/mL | 15 units | 30 units | 60 units |
| 10 mg | 1 mL | 10 mg/mL | 2.5 units | 5 units | 10 units |
| 10 mg | 2 mL | 5 mg/mL | 5 units | 10 units | 20 units |
| 10 mg | 3 mL | 3.33 mg/mL | 7.5 units | 15 units | 30 units |
Read across a row: the same mg takes more units when the concentration is lower. Compare the 5 mg vial with 1 mL (10 units for 0.5 mg) and with 3 mL (30 units for 0.5 mg). The amount of semaglutide in the vial doesn't change; only the volume it's spread through does. For all the trade-offs of different diluent volumes, see how much BAC water for a 10 mg vial, and for larger vials, 30, 40 and 60 mg vials.
Worked examples (math only)
Hypothetical numbers that illustrate the arithmetic. None is a dose suggestion.
Example 1: 5 mg vial, 2 mL added
- Concentration: 5 ÷ 2 = 2.5 mg/mL
- Drawing 0.5 mg: 0.5 ÷ 2.5 = 0.2 mL
- Units: 0.2 × 100 = 20 units
- Check: 20 ÷ 100 × 2.5 = 0.5 mg ✓
Example 2: the same vial, 1 mL added
- Concentration: 5 ÷ 1 = 5 mg/mL
- 0.5 mg: 0.5 ÷ 5 = 0.1 mL = 10 units
Same powder, same 0.5 mg, half the units. Drawing the Example 1 count of 20 units from this vial would give 1 mg, twice the amount.
Example 3: 10 mg vial, 2 mL added
- Concentration: 10 ÷ 2 = 5 mg/mL
- 0.25 mg: 0.25 ÷ 5 = 0.05 mL = 5 units
Example 4: from units back to mg
A vial is 2.5 mg/mL and the instructions say 20 units. 20 units is 0.2 mL, and 0.2 × 2.5 = 0.5 mg. Run this whenever you have a unit count and want to know the mg behind it, and confirm that mg with your prescriber or pharmacist.
Example 5: don't round the concentration
A 5 mg vial with 3 mL added is 5 ÷ 3 = 1.6667 mg/mL, which is often shown as 1.67. Draw 0.5 mg: 0.5 ÷ 1.6667 = 0.3 mL = 30 units. Rounding the concentration to 1.67 first gives 29.9, which looks like a different answer. Keep the full division until the last step, then round the units to what your syringe can show.
Example 6: a new vial with a different concentration
Last vial: 5 mg/mL, and 1 mg was 20 units. New vial: 10 mg/mL, so 1 mg is 1 ÷ 10 = 0.1 mL = 10 units. The same 20 units from the new vial would be 2 mg. Whenever a concentration changes, recalculate; see New vial, different concentration, which includes a vial-switch calculator.
How long does reconstituted semaglutide last?
Use it by the beyond-use date your pharmacy or the label gives, and never past it. 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. Don't freeze a vial, and don't use one that looks cloudy, discolored or has particles. Wegovy and Ozempic pens have their own storage rules on their labels; see the storage guide and the 28-day rule for mixed vials.
Common mistakes
- Adding water to a vial that's already liquid.
- Using a different volume than the instructions specify and not recalculating the units.
- Treating "5 mg" as the concentration. It's the total in the vial; the concentration is mg ÷ mL.
- Reusing an old unit count with a new vial or a new mix. Always recalculate when the concentration changes.
- Reading the wrong syringe scale. The math here assumes U-100; check the box and barrel, and see insulin syringe units explained.
- Shaking the vial hard instead of swirling it.
- Keeping a mixed vial past its use-by date to use up what's left.
- Not writing down the mix date, the volume added and the concentration.
Tracking it in Caliper
Caliper saves each vial with its concentration (or the powder's mg and the diluent you added), its reconstitution or opened date and its use-by date. Each dose is logged with one tap along with where you injected it, reminders tell you when the next one is due, and your provider can see the timeline in a PDF report. Caliper never recommends or calculates a dose; it records what you and your provider decide. Use the calculator above for the arithmetic and your pharmacy's instructions for the amount.
FAQ
Does semaglutide need to be reconstituted?
Only if your vial holds freeze-dried powder. Wegovy and Ozempic come as ready-to-use pens or prefilled syringes, and a vial that already contains liquid shouldn't have anything added to it.
How do you reconstitute semaglutide?
Follow your pharmacy's or label's instructions for the diluent and volume. In general: wash your hands, swab both stoppers, add the diluent slowly down the inside wall of the vial, swirl gently until clear without shaking, then label the vial with the date, the volume added and the concentration.
How many units is 0.5 mg of semaglutide?
It depends on the concentration. At 2.5 mg/mL, 0.5 mg is 0.2 mL, which is 20 units on a U-100 syringe; at 5 mg/mL it's 10 units. The formula is mg ÷ concentration (mg/mL) × 100. Math only; your amount comes from your prescriber.
How do I calculate the concentration of a semaglutide vial?
Divide the mg in the vial by the mL of diluent added. A 5 mg vial with 2 mL is 2.5 mg/mL, and the same vial with 1 mL is 5 mg/mL.
Does adding more water change how much semaglutide is in the vial?
No. The amount of semaglutide in the vial stays the same. More water makes a weaker concentration, so the same mg takes more units; less water makes a stronger concentration and takes fewer units.
How long does reconstituted semaglutide last?
Use it by the beyond-use date on the label or from your pharmacy. 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.
Can I draw the same number of units from a new vial?
Only if the concentration is exactly the same. If it changed, recalculate from your prescribed mg and confirm with your pharmacy. Our new-vial guide has a vial-switch calculator.
Should I shake the vial to dissolve the powder?
No. Swirl or roll it gently; shaking hard is the common mistake. If it doesn't become clear, or it looks cloudy or has particles, don't use it and ask your pharmacist.
The takeaway
Check whether you need to mix anything at all. Pens and liquid vials don't need it. For a powder, use the diluent and volume you were given, write the concentration on the vial, and run the three steps: concentration, mL, units. Check the answer backwards, and recalculate every time a vial or a mix changes.
Sources
- Wegovy (semaglutide) prescribing information, Novo Nordisk
- Ozempic (semaglutide) prescribing information, Novo Nordisk
- FDA, Dosing errors associated with compounded injectable semaglutide products
- 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 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. 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.