Reconstitution is the process of mixing a powdered peptide with bacteriostatic water to create a usable liquid. The procedure is mechanically simple — but the math of "how many units do I draw?" is where almost everyone stumbles the first time. This guide covers both: the procedure and the math, with the actual numbers shown.
What you'll need
Three items, in front of you, before you open anything:
- A 5mg vial of semaglutide (the lyophilized powder)
- Bacteriostatic water — usually a 30mL multi-dose vial
- A 100-unit insulin syringe (1mL / 1cc, with U-100 markings)
Optional but useful: alcohol swabs, a notebook, and a steady, well-lit surface. The whole procedure takes about three minutes once you've done it once or twice.
Step 1 — Add the water
The peptide vial arrives sealed under vacuum, with the lyophilized (freeze-dried) powder at the bottom. You're going to inject a measured volume of bacteriostatic water into it through the rubber stopper. The vacuum will help pull most of it in.
How much water? That depends on the concentration you want. The most common choices for a 5mg vial:
- 1 mL → 5000 mcg/mL (very concentrated, tiny units)
- 2 mL → 2500 mcg/mL (most common, easy unit math)
- 5 mL → 1000 mcg/mL (dilute, larger draws)
For this guide we'll use 2 mL — the most common choice. That gives a final concentration of 2500 mcg/mL.
Step 2 — Swirl, don't shake
After injecting the water, the powder will start dissolving. Tilt and swirl the vial gently for ~30 seconds until the solution is clear. Do not shake — shaking creates foam and can denature the peptide.
If you see undissolved particles after a minute, let the vial sit upright for a few more minutes. They usually dissolve on their own.
Step 3 — Calculate your units
Here's where the math comes in. You know your concentration (2500 mcg/mL) and your target dose. Let's say you've been prescribed 250 mcg per shot.
The four-step conversion:
- Total mcg in vial:
5 mg × 1000 = 5000 mcg - Concentration:
5000 mcg ÷ 2 mL = 2500 mcg/mL - Volume per dose:
250 mcg ÷ 2500 mcg/mL = 0.1 mL - Units on a 100u/mL syringe:
0.1 mL × 100 = 10 units
So you draw 10 units on your insulin syringe. That's the only number you actually need before you inject.
Step 4 — Draw and store
Once you know the units, draw your dose into the syringe, then return the vial to the fridge. Refrigerated at 2–8°C, a reconstituted vial of semaglutide is good for ~28 days thanks to the 0.9% benzyl alcohol in the BAC water.
After 28 days, discard whatever's left and start over with a fresh vial.
A sanity check
A 5mg / 2mL / 250mcg vial gives you 20 doses total (5000 mcg ÷ 250 mcg = 20). At a once-weekly cadence, that's about five months of supply. The calculator on this site will compute all of these numbers for you — vial life, doses left, units per shot — but doing the math once by hand is a useful sanity check.