Reference
Peptide reconstitution
Peptides ship as a freeze-dried powder and have to be dissolved before use. The arithmetic is simple, and getting it wrong is the most common way people end up taking ten times what they intended. Here is how it works.
Reconstitution calculator
Converts a vial size and diluent volume into concentration, then converts a dose you enter into millilitres and syringe units. It does not suggest a dose — the dose is yours, and the arithmetic is ours.
Enter a vial size and diluent volume to see the concentration.
Syringe markings matter. U-100 and U-40 syringes look nearly identical and are marked differently — reading one as the other is a 2.5× error. Check the barrel before you draw.
This tool converts units. It is not a statement that any dose is safe, effective, or appropriate for you, and most peptides have no established human dose at all. Where published studies exist, we list what they used in the study dose reference. Not medical advice — talk to a licensed prescriber.
What reconstitution is
Peptides are supplied lyophilised — freeze-dried into a powder or a thin film at the bottom of the vial. Dry, they are relatively stable. In solution, they are not, which is why they are shipped this way and why the solution has a limited life once mixed.
Reconstitution is dissolving that powder in a sterile liquid. The powder has a fixed mass — a 5 mg vial contains 5 mg whether you add 1 mL or 3 mL. What changes is the concentration, and therefore how much liquid corresponds to a given dose.
The maths
One equation does all of it:
concentration = vial contents ÷ diluent volume
A 5 mg vial reconstituted with 2 mL gives 2.5 mg/mL, or 2500 mcg/mL. To find the volume for a dose: volume = dose ÷ concentration. A 250 mcg dose at 2500 mcg/mL is 0.1 mL.
More diluent does not mean a weaker dose — it means the same dose occupies more liquid. Diluting further makes small doses easier to measure accurately, which is the main reason to do it.
Syringe units — where the errors happen
Insulin syringes are marked in units, not millilitres. On a U-100 syringe, 100 units = 1 mL, so 0.1 mL is 10 units. This is the standard in most of the world.
U-40 syringes also exist, where 40 units = 1 mL. They look almost identical. Reading a U-40 syringe as though it were U-100 gives a 2.5× error in either direction. Check the barrel every time.
The other frequent error is mcg versus mg: 1 mg = 1000 mcg. Study doses for peptides are often written in mcg and vial contents in mg, so the two get mixed in the same calculation. The calculator above flags it when a dose comes out larger than the vial, which is the usual signature of this mistake.
Bacteriostatic versus sterile water
Bacteriostatic water is sterile water containing roughly 0.9% benzyl alcohol, a preservative that inhibits bacterial growth. It is what allows a vial to be entered more than once, and is the reason a reconstituted vial can be kept for a period rather than used immediately.
Sterile water for injection has no preservative. It is intended for single use — once the vial is entered there is nothing suppressing contamination.
Benzyl alcohol is not appropriate for everyone; it is contraindicated in neonates, and some people react to it. This is one of several reasons the choice of diluent is a clinical decision rather than a preference.
A worked example from an actual label
Tesamorelin (Egrifta SV) is one of the few peptides in this category with an FDA-approved label, and its instructions are worth reading because they differ from the gray-market default in two instructive ways.
The label directs reconstituting one 2 mg vial with 0.5 mL of Sterile Water for Injection — the diluent supplied with it — giving 2 mg per 0.5 mL, or 4 mg/mL. The dose is 1.4 mg, which works out to 0.35 mL. You mix it by rolling the vial gently for 30 seconds, and you do not shake it.
The two departures from common practice: it specifies sterile, not bacteriostatic, water, and it instructs you to inject immediately and discard the remainder — no multi-day storage of the reconstituted vial at all. Where a manufacturer has run the stability work, the answer is often more conservative than the one circulating in forums. See the tesamorelin brief for the approved indication and the evidence behind it.
Storage and stability
As a general pattern, lyophilised peptide is stored cold and dry, and reconstituted peptide is refrigerated and has a much shorter usable life — often measured in weeks rather than months. Most are sensitive to light, heat, and freeze-thaw cycles, and repeated warming and cooling degrades them.
Specific figures vary substantially by peptide, formulation, and manufacturer, and the only reliable numbers are the ones on the product documentation for what you actually have. For the approved medicines in this category — semaglutide, tirzepatide, tesamorelin, bremelanotide — the approved label carries validated storage and in-use stability data, and that label is the authority, not a general rule of thumb.
What this page does not tell you
It does not tell you what dose to take. For most peptides discussed online there is no established human dose, because the human trials that would establish one were never run. Where studies do exist, we record what they used, with the study attached, in the study dose reference — those are doses used in published research, not recommendations.
It also does not cover injection technique, site selection, or sterile procedure. Those are clinical skills, they carry real infection risk when done wrong, and a web page is the wrong place to learn them.