How to Reconstitute Peptides: Step-by-Step With the Math

What does it mean to reconstitute a peptide?

To reconstitute a peptide is to add a liquid, called a diluent, to a vial of freeze-dried powder so it becomes an injectable solution. Freeze-drying is called lyophilization. Pharmaceutical stability reviews treat the dried state and a water solution as separate formulation problems, and a real label shows why.^[5] The FDA-approved peptide Egrifta WR is stored as a dry powder at 20 to 25 degrees Celsius, but the label says to discard the solution 7 days after mixing.^[2] One presentation of that product comes with its own diluent, Bacteriostatic Water for Injection, USP, and the label says to use only the diluent provided.^[2] The lesson is that the label for your product decides the diluent, the volume, and the discard time. Many vials sold as research peptides are not FDA-approved, and the FDA has reported dosing errors from mis-measured vials.^[4] If your product has no label, ask a pharmacist before you mix anything.

How do you reconstitute a peptide step by step?

The steps below are general aseptic technique drawn from the Egrifta WR label, the bacteriostatic water label, and CDC injection safety guidance. They do not replace the instructions that came with your product. In short, you read the label, clean the stoppers, draw a measured volume of diluent with a new sterile syringe, add it slowly to the powder, swirl without shaking, and check that the solution is clear. Then you work out concentration and the syringe line for your prescribed dose, label the vial, and discard sharps safely.^[2][3] The Egrifta WR label says to swirl the vial and not shake it, and to use the solution only if it is clear, colorless, and free of particles.^[2] The bacteriostatic water label adds that it should not be used unless the solution is clear and the seal is intact.^[1] FDA guidance says used sharps should go straight into a sharps disposal container.^[6] Ask your pharmacist or prescriber to watch you do it once.

1. Read the label and gather suppliesCheck your product's instructions for the diluent and volume. You need the vial, the diluent, a new sterile syringe and needle, alcohol swabs, and a sharps container.
2. Wash your hands and clean a flat surfaceWash your hands and wipe down the area where you will work.
3. Inspect both vialsCheck that the seals are intact, the diluent is clear, and nothing is expired. Do not use bacteriostatic water unless the solution is clear and the seal is intact.
4. Wipe both rubber stoppersWipe each stopper with a fresh alcohol swab and let it dry.
5. Draw the diluentUse a new sterile syringe and needle to draw the volume of diluent your label calls for. This page uses 2 mL as its example.
6. Add the diluent slowlyInject the diluent into the peptide vial slowly, aimed at the glass wall instead of straight onto the powder, unless your label says otherwise.
7. Swirl gently, do not shakeMove the vial in a circle until the powder dissolves. Do not shake it.
8. Check the solutionUse it only if it is clear and free of particles, as the label for your product describes. If it is cloudy, ask a pharmacist.
9. Work out concentration and drawDivide vial mg by mL added, then divide your prescribed dose by that concentration, then multiply by 100 for U-100 units. For 5 mg in 2 mL, that is 2.5 mg per mL, so 0.25 mg is 0.10 mL, or 10 units.
10. Label, store, and disposeWrite the date and concentration on the vial, store it as the product label says, and put the used needle and syringe in a sharps container.

How much bacteriostatic water do you add?

There is no universal amount. Approved products state their own: the Egrifta WR label calls for 1.3 mL of diluent per vial.^[2] For a vial with no label, the volume you add only sets the concentration, and it does not change how much peptide is in the vial. A 5 mg vial mixed with 2 mL is 2.5 mg per mL. The same vial mixed with 3 mL is 1.667 mg per mL. More water therefore means a larger draw, in more units, for the same dose, and not a weaker dose. Bacteriostatic water is sterile water for injection that contains 0.9 percent or 1.1 percent benzyl alcohol, and its label says it is used only for diluting or dissolving drugs according to the drug maker's instructions.^[1] Pick the volume your product label or pharmacist gives you, then use the calculator to confirm the syringe units.

How do you calculate peptide concentration and syringe units?

Three formulas cover it. Concentration in milligrams per milliliter equals vial milligrams divided by milliliters of diluent. Volume to draw in milliliters equals the dose in milligrams divided by the concentration. Syringe units equal that volume multiplied by the units per milliliter on the syringe, which is 100 on a U-100 insulin syringe. The table below shows seven worked rows, computed with the same function the PepTrac calculator uses. They are arithmetic examples, not dosing guidance. The doses are placeholders to show the method. Your prescriber sets the dose, and these numbers do not suggest one. Mixing up milligrams, milliliters, and units causes real harm. The FDA described patients directed to draw 5 units from a compounded semaglutide vial who drew 50 units, and it reported patients taking five to 20 times the intended dose.^[7] Always check the scale printed on your syringe.

What is the difference between U-100 and U-40 syringes?

The number after the U is how many unit marks the syringe has per milliliter. A U-100 syringe has 100 marks per mL, so one unit is 0.01 mL, and 0.10 mL reads as 10 units. A U-40 syringe has 40 marks per mL, so the same 0.10 mL reads as 4 units. Units are a measure of volume on the barrel, not an amount of peptide, which is why the same draw reads differently on each scale. Reading a U-40 syringe as if it were U-100 produces a 2.5 times error, because 100 divided by 40 is 2.5. The FDA has reported that confusion over units, milligrams, and milliliters led patients using compounded semaglutide to take five to 20 times the intended dose, and some needed medical care.^[7] Check the scale printed on the barrel before you read any line, and say which scale you use when you ask a pharmacist for help.

What if your dose falls between syringe lines or does not fit?

A dose between two printed lines cannot be drawn exactly. A 5 mg vial mixed with 2 mL of water has a concentration of 2.5 mg per mL, and a 0.25 mg dose is exactly 10 units. If you mix the same vial with 2.5 mL instead, the same dose is 12.5 units, which falls between lines, and rounding to 13 units would deliver slightly more. The arithmetic is simple: the water volume sets where the dose lands. A 0.25 mg dose lands on 20 units from a 5 mg vial with 4 mL of water. Whether to change a volume is for your pharmacist or prescriber. A dose can also overflow the barrel: 60 units cannot fit in a 50-unit barrel, and less water would shrink the volume. What is appropriate for your product is a question for your pharmacist or prescriber. The PepTrac calculator warns about both cases, and it shows what a rounded draw would actually deliver.

How long does a reconstituted peptide last?

It depends on the product, and for unapproved research peptides no validated figure exists, so none is printed here. The one approved peptide we checked, Egrifta WR, says to discard unused solution 7 days after mixing.^[2] The bacteriostatic water label says not to store reconstituted solutions of drugs for injection unless the maker of the drug directs it.^[1] The CDC rule of 28 days applies to the water vial itself: once a multi-dose vial is needle-punctured, it should be dated and discarded within 28 days unless the manufacturer states another date.^[3] That limit is about the diluent bottle and does not tell you how long a mixed peptide keeps. The FDA also says compounded drugs are not FDA-approved and are not reviewed for safety, effectiveness, or quality.^[4] Write the mixing date on the vial and ask your pharmacist how long yours is good for.