How to Reconstitute Semaglutide, Step by Step

Before anything else, settle which vial is in front of you. Two of the three things sold under this name are not reconstituted at all — they arrive as finished liquid, and adding water to them is a mistake rather than a step. Only the lyophilized vial is a powder waiting for a volume, and that volume decides one thing only: how the amount reads on the barrel. This is general information about mechanics and arithmetic, not medical advice.

Three formats, and only one of them is a powder

The approved injectables — Ozempic and Wegovy — ship as ready-to-use solution in prefilled pens. Ozempic uses 3 ml pens at 0.68, 1.34 and 2.68 mg/ml; Wegovy is presented at 1.34 mg/ml among others. They are already at concentration, they carry their own excipients, and they are not intended to meet bacteriostatic water at any point.

The third format is the one this page is about: a sealed vial holding a white freeze-dried cake, commonly labelled 2, 3, 5 or 10 mg, occasionally 15. Semaglutide is a 31-residue peptide built on the GLP-1 backbone, acylated with a C18 diacid chain, at about 4,114 g/mol — heavy for a peptide, and stable in the dry state, which is why it travels as a cake rather than as a liquid.

Where that vial came from is worth knowing. The FDA removed semaglutide from its drug shortage list on 21 February 2025; the enforcement window for 503A pharmacies closed on 22 April 2025 and for 503B outsourcing facilities on 22 May 2025, which returned compounded semaglutide to the ordinary limits of those two sections. Material labelled for research is not a medicine and is not supplied for use in people.

How to reconstitute semaglutide, step by step

The method is the ordinary one. The care goes into the decision made before the first puncture.

Let the vial reach room temperature. Ten to fifteen minutes out of the fridge. Cold glass and cold water only slow the cake down.
Fix the water volume first. Water cannot be taken back out. Bacteriostatic water is sterile water carrying 0.9% benzyl alcohol as a preservative, which is what makes repeated entries workable; plain sterile water carries none.
Swab both stoppers with fresh alcohol wipes and let them air dry.
Draw the water with a syringe graduated in millilitres. For one or two millilitres an insulin barrel is the wrong instrument.
Run the stream down the glass. Angle the needle so the water slides down the inside wall instead of landing on the cake.
Swirl, never shake. Roll the vial slowly between your fingers until the cake is gone. Foam is a sign of force, not of progress.
Inspect against white paper. The solution should be clear and colourless, with nothing floating in it.
Label it before you put it away. The date of first puncture, and the concentration you just created.

Choosing the water so the units stay readable

Once it is liquid, the rest is division. The milligrams printed on the vial divided by the millilitres of water gives the concentration, and one mark on a U-100 insulin barrel is 0.01 ml — always, whatever is dissolved in it.

mg in vial ÷ ml of water = mg/ml · ÷ 100 = mg per unit · amount ÷ mg per unit = units

Semaglutide is where this matters more than usual, because the amounts involved are small fractions of a milligram and the water is the only lever on how they read. Purely to run the maths, take 0.25 mg as the amount and watch it move: a 5 mg vial with 2 ml of water sits at 2.5 mg/ml, so a mark holds 0.025 mg and the amount lands on 10 units. The same vial with 1 ml is 5 mg/ml, a mark holds 0.05 mg, and it lands on 5. A 2 mg vial with 2 ml is 1 mg/ml, a mark holds 0.01 mg, and it lands on 25.

Nothing about those three lines changes how much peptide is administered. The same 0.25 mg is drawn every time. What changes is the precision you can actually see: misreading by half a mark costs 10% at five marks and 2% at twenty-five. Weaker mixes are easier to read — until the volume gets uncomfortable to inject, or runs past the 100 marks on the barrel.

The amount itself comes from a qualified clinician. This arithmetic only translates it into a position on a barrel, and it has to be redone for every vial and every water volume. That conversion is what the Vialog calculator runs, so the division is not repeated by hand at two in the morning.

Storage, and whose clock you are on

Dry is the stable state. Sealed, cold, dark and away from moisture, the powder keeps far longer than anything in solution, which is exactly why it was freeze-dried in the first place. Once mixed, refrigerate at 2 to 8 °C — in the body of the fridge, not the door, where the temperature swings every time it opens — and do not freeze it.

After that, the limiting clock usually belongs to the water rather than to the peptide. Bacteriostatic water for injection is labelled for use within 28 days of first puncture, and that clock starts when the stopper is pierced, not when the vial starts running low. Write the date on the glass. Write the concentration next to it, because the milligrams printed on the label stopped being the whole story the moment water went in.

This is general educational information about reconstitution mechanics and unit arithmetic. It is not medical advice and not a dosing recommendation. What to administer, and whether to administer anything at all, is a decision for a qualified healthcare professional.

Frequently asked

How much bacteriostatic water goes into a 5 mg semaglutide vial?

There is no fixed answer, because the volume is a readability choice rather than a property of the vial. Adding 2 ml gives 2.5 mg/ml, so one mark on a U-100 barrel holds 0.025 mg; adding 1 ml gives 5 mg/ml and 0.05 mg per mark. The right volume is the one that puts the amount your clinician specified on a whole mark you can read without squinting.

Do Ozempic and Wegovy pens need to be reconstituted?

No. Both are supplied as ready-to-use solution already at concentration — Ozempic in 3 ml pens at 0.68, 1.34 or 2.68 mg/ml, Wegovy at 1.34 mg/ml among others. Bacteriostatic water has no role there. Reconstitution applies only to a vial that arrives as a dry lyophilized cake.

Can sterile water be used instead of bacteriostatic water?

It dissolves the powder identically, but it carries no preservative, so the vial has no protection against contamination once it has been entered. Bacteriostatic water contains 0.9% benzyl alcohol for exactly that reason, which is what makes a multi-entry vial workable. Sterile water is generally treated as single-use.

How many units is a given amount after reconstitution?

It depends entirely on the water, because a unit is a volume and not an amount. One mark on a U-100 barrel is 0.01 ml whatever it contains, so divide the vial milligrams by the millilitres added, divide that by 100 to get milligrams per mark, then divide the amount by that figure. The same amount can land on five marks or twenty-five depending on nothing but the mixing.

How long does reconstituted semaglutide keep in the fridge?

The practical limit is usually the water rather than the peptide: bacteriostatic water for injection is labelled for use within 28 days of the first puncture. Keep the vial at 2 to 8 °C, in the body of the fridge rather than the door, and do not freeze it. Date the vial at the moment the stopper is first pierced, not later.

Keep reading: How to reconstitute a peptide · How many units do I draw?.