You have a 2 mg vial of lyophilised semaglutide and a vial of bacteriostatic water in front of you, and nothing on either label tells you how much of one goes into the other. Every calculator you find online returns a different number, and this is not a compound where you want to guess.
The good news is that the maths is simple once you see it laid out, and the procedure takes about three minutes. This guide covers exactly how much water to add, how to convert a milligram dose into units on an insulin syringe, how to inject it, and how to store it so the vial lasts its full window.
First: Pen or Vial?
There are two formats and only one of them needs any of this.
- Pre-filled pen: Already in solution and dose-metered. You dial the dose, attach a needle, inject. No reconstitution, no calculation. Skip to the injection section below.
- Lyophilised vial: A freeze-dried white powder or cake at the bottom of a sealed glass vial. Inert and stable in this state, but it must be dissolved before use. This is what the rest of this guide is about.
If the powder looks like it has shifted or coats the side of the vial, that is normal — it happens in transit and does not affect the product.
Why Bacteriostatic Water, Not Sterile Water
These are not interchangeable, and the difference decides whether your vial lasts four weeks or one use.
| Bacteriostatic water | Sterile water | |
|---|---|---|
| Contains | Water plus 0.9% benzyl alcohol (a preservative) | Water only |
| Multi-dose use | Yes — inhibits bacterial growth between draws | No — single use only |
| Shelf life once mixed | Approximately 28 days refrigerated | Discard immediately after use |
| Correct choice here | Yes | Only if you are using the entire vial at once |
Since a semaglutide vial is drawn from weekly across several weeks, bacteriostatic water is the correct solvent. Do not use tap water, saline, or anything else.
→ Bacteriostatic Water — Hilma Biocare
The Reconstitution Procedure
You will need: your peptide vial, bacteriostatic water, a 3 mL syringe with a drawing needle for the water, alcohol swabs, and insulin syringes for dosing.
- Let the vial reach room temperature. Take it out of the fridge fifteen minutes beforehand. Cold vials dissolve more slowly.
- Swab both stoppers. Alcohol wipe on the peptide vial and the bacteriostatic water vial. Let them air dry — do not fan or blow on them.
- Draw your water. Pull the volume you calculated (see the next section) into the syringe. Invert the water vial, draw, tap out air bubbles.
- Inject down the side wall. Insert the needle at an angle so the water runs down the inside glass wall rather than jetting directly onto the powder. Release slowly and let the vacuum in the vial draw it in. Semaglutide is a peptide — a direct high-pressure stream onto the cake can damage it.
- Swirl gently. Never shake. Roll the vial between your palms or swirl it slowly. Shaking denatures peptides and creates foam. The solution should go completely clear within a minute or two.
- Inspect. Clear and colourless with no visible particles. If it stays cloudy or you can see floating matter, do not use it.
- Label and refrigerate. Write the concentration and the reconstitution date on the vial. You will not remember either in three weeks.
The Mixing Maths
One formula governs everything:
Concentration (mg/mL) = total mg in the vial ÷ mL of bacteriostatic water added
And one conversion, because you will be dosing with an insulin syringe:
On a standard U-100 insulin syringe, 1 mL = 100 units. So 0.5 mL is 50 units, 0.25 mL is 25 units.
For a 2 mg vial, adding 1 mL of bacteriostatic water gives you 2 mg/mL. This is the most practical ratio — it keeps every dose in the titration ladder at a readable volume on the syringe.
| Dose | Volume | Units on a U-100 syringe |
|---|---|---|
| 0.25 mg | 0.125 mL | 12.5 units |
| 0.5 mg | 0.25 mL | 25 units |
| 1.0 mg | 0.5 mL | 50 units |
| 1.7 mg | 0.85 mL | 85 units |
| 2.0 mg | 1.0 mL | 100 units |
If you prefer larger, easier-to-read volumes at the low end of the ladder, add 2 mL instead for a 1 mg/mL concentration. Then 0.25 mg is 25 units, 0.5 mg is 50 units, and 1.0 mg is a full 100-unit syringe. The trade-off is that anything above 1 mg needs two injections, so this ratio suits the early titration weeks rather than the later ones.
Sanity check before every injection: units drawn ÷ 100 × concentration = your dose in mg. If that number is not what you intended, stop and recalculate.
How Long a Vial Lasts
A 2 mg vial covers roughly the first eight weeks of the standard titration:
| Weeks | Weekly dose | Total used |
|---|---|---|
| 1–4 | 0.25 mg | 1.0 mg |
| 5–8 | 0.5 mg | 2.0 mg |
Note the constraint this creates: reconstituted semaglutide is good for about 28 days, so a single 2 mg vial cannot cover eight weeks in one mix. Reconstitute half the vial's worth at a time, or plan on the 5 mg pen once you reach the higher doses.
Injection Technique
Semaglutide is injected subcutaneously — into the fat layer beneath the skin, not into muscle.
- Needle: A 29–31 gauge insulin syringe, 8 mm or shorter. These are short and fine enough that most people feel very little.
- Sites: Abdomen (staying at least 5 cm away from the navel), outer thigh, or the back of the upper arm. The abdomen is the most convenient and the most consistent.
- Rotation: Move site every week. Repeatedly injecting the same spot causes lipohypertrophy — firm fatty lumps that also absorb unpredictably.
- Technique: Swab the skin and let it dry. Pinch a fold of skin, insert at 90 degrees (45 degrees if you are lean), inject slowly over a few seconds, wait a moment, withdraw, release the pinch.
- Timing: Same day each week. The weekly half-life means the time of day matters very little — pick whatever you will actually remember.
A small bead of blood or a minor bruise is normal. Persistent redness, swelling or warmth at the site is not, and warrants attention.
Storage and Shelf Life
| State | Storage | Shelf life |
|---|---|---|
| Unreconstituted powder | Refrigerated, 2–8°C, protected from light | Until the labelled expiry |
| Reconstituted solution | Refrigerated, 2–8°C, protected from light | Approximately 28 days |
| Bacteriostatic water | Room temperature | Approximately 28 days after first puncture |
Do not freeze reconstituted solution — freeze-thaw cycles degrade peptides. Keep the vial in its carton or somewhere dark; light exposure degrades it over time. Brief periods at room temperature during handling are fine, but do not leave it out overnight.
Five Mistakes That Ruin a Vial
- Shaking instead of swirling. The most common error. It denatures the peptide and you lose potency without any visible sign beyond foaming.
- Using sterile water for a multi-dose vial. No preservative means contamination risk from the second draw onward.
- Injecting water directly onto the powder at pressure. Aim for the glass wall and let it run down.
- Not labelling the vial. Three weeks later you have a clear liquid of unknown concentration and unknown age. It gets thrown away.
- Confusing mL with units. The single most dangerous mistake here — a fourfold overdose is easy if you draw 50 units thinking it is 0.5 mg at the wrong concentration. Run the sanity check every time.
→ Semaglutide vials, pens and bacteriostatic water are stocked in our Peptides category. Every batch is third-party tested.
Frequently Asked Questions
Can I add more or less water than recommended?
Yes — the amount of water only sets the concentration, not the total drug in the vial. Adding 1 mL to a 2 mg vial gives 2 mg/mL; adding 2 mL gives 1 mg/mL. Either works as long as you recalculate your units accordingly and write the concentration on the vial. What you cannot do is change the total milligrams available.
What if I inject an air bubble?
A small air bubble in a subcutaneous injection is harmless — the volumes involved are far too small to cause a problem. Tap the syringe and expel obvious bubbles for accuracy of dosing rather than for safety, since a bubble displaces solution and means you draw slightly less drug than intended.
My solution looks slightly cloudy. Is it ruined?
Give it a few more minutes of gentle swirling — undissolved powder is the usual cause and it clears. If it stays cloudy, or you see particles, strands or discolouration after full dissolution, discard the vial. Do not inject anything you cannot see clearly through.
Do I need to swab the stopper every single time?
Yes. The bacteriostatic preservative inhibits bacterial growth in the solution, but it does not sterilise the rubber stopper you are pushing a needle through. Swab before every draw and use a fresh needle each time.
Can I reconstitute the whole vial and freeze portions?
Not recommended. Freezing and thawing degrades peptides, and the benefit is minimal given the 28-day refrigerated window. If your dosing schedule will not consume a full vial in that time, reconstitute a smaller portion of the powder and leave the rest dry.
For the full picture on dosing schedules, expected results and side effect management, see the complete semaglutide guide.
Disclaimer: This content is for informational and research purposes only. This does not constitute medical advice. Semaglutide is a prescription medication in most jurisdictions. Individual results may vary. Consult a qualified healthcare professional before use.
