How to Reconstitute Peptides: Step-by-Step, BAC Water Amounts & Chart by Compound

Key takeaways

Reconstitution means dissolving a freeze-dried (lyophilized) peptide powder in a measured volume of sterile liquid so it can be injected. The water volume sets the concentration; it does not change how much peptide is in the vial.
Use the diluent and the volume on your pharmacy label. Bacteriostatic water (0.9% benzyl alcohol) is the standard for multi-dose vials; sterile water has no preservative and is for single use.
Inject the water slowly down the inside wall of the vial and swirl gently. Shaking can denature the peptide and foam the solution.
Date the vial. The CDC and USP convention for an opened multi-dose vial is 28 days, refrigerated, unless the label gives a different beyond-use date, and never past the expiration date.
Clear and colorless is the only acceptable result. Cloudy, discolored or particulate solutions go back to the pharmacy, not into you.

What reconstitution is, and when you do not need to do it

Many injectable peptides are shipped as a lyophilized powder because the dry form is far more stable than a solution. Before the first dose you add a precise volume of sterile liquid, the powder dissolves, and the vial becomes a multi-dose solution you draw from with an insulin syringe for the next few weeks.

Not every injectable needs this step. Most compounded semaglutide and tirzepatide, and all brand GLP-1 pens, are dispensed as ready-to-use solutions. NAD+ is usually dispensed as a solution as well. Sermorelin, tesamorelin and most growth hormone secretagogues are dispensed as powder. If your vial already contains liquid, there is nothing to reconstitute, and adding water would dilute your medication.

This guide covers pharmacy-dispensed medication. A prescription vial comes with a label that states the amount of peptide (for example, 6 mg), the diluent to use, the volume to add and the beyond-use date. Those instructions override anything on this page.

What you need

Item	Why it matters
The peptide vial (lyophilized powder), at room temperature	Cold powder can dissolve unevenly; a vial straight from the refrigerator also forms condensation on the stopper
Bacteriostatic water for injection (BAC water), or the diluent your label specifies	The preservative is what allows repeated withdrawals from the vial
A mixing syringe (1 to 3 mL) with a needle, or a 1 mL insulin syringe for small volumes	You need to measure the water accurately; a syringe is the only way
Alcohol swabs (70% isopropyl)	Wipe both vial stoppers before every puncture
A sharps container	Needles and syringes go in an FDA-cleared sharps container, not the trash
A marker or label	Write the date of reconstitution and the concentration on the vial
Clean, dry surface and clean hands	Most contamination comes from hands and surfaces, not the air

Bacteriostatic water versus sterile water

The two look identical and are both sold in small vials. They are not interchangeable.

	Bacteriostatic water for injection	Sterile water for injection
Preservative	0.9% benzyl alcohol (some products 1.1%)	None
Container	Multiple-dose vial, usually 30 mL, designed for repeated withdrawals	Single-dose; once punctured it must be used or discarded
Typical use	Reconstituting vials you will draw from for days or weeks	Single-use reconstitution, or when benzyl alcohol must be avoided
Who must not use it	Neonates. The label carries a warning that solutions containing benzyl alcohol must not be used in newborns	Not for fluid replacement; same for both
Can it be used alone	No. It is a diluent, never an injection by itself	Same

The Hospira label describes bacteriostatic water as a multiple-dose container "from which repeated withdrawals may be made to dilute or dissolve drugs for injection," to be used "according to instructions of the manufacturer of the drug to be administered." That last clause is the point: the drug label, not the water label, decides the volume.

How much water to add: the math

Three numbers decide every dose: how much peptide is in the vial (mg), how much water you add (mL), and the dose you want (mcg). The relationships:

Concentration (mcg per mL) = peptide mg × 1,000 ÷ water mL
Volume per dose (mL) = dose mcg ÷ concentration
Insulin-syringe units = volume mL × 100

More water means a lower concentration and more units per dose. For small doses that is a feature: 5 units is hard to read accurately on a syringe, 25 units is easy. The limits are the capacity of the vial (most peptide vials hold 2 to 3 mL comfortably) and the volume you are willing to inject (most people prefer under 0.5 mL per subcutaneous injection).

Reconstitution chart

The table shows what concentration common vial sizes give with common water volumes, and the syringe units that correspond to three example doses. The doses are arithmetic examples, not recommendations; your prescription specifies the dose.

Vial	Water added	Concentration	100 mcg	250 mcg	500 mcg
2 mg	1 mL	2,000 mcg/mL	5 units	12.5 units	25 units
2 mg	2 mL	1,000 mcg/mL	10 units	25 units	50 units
5 mg	1 mL	5,000 mcg/mL	2 units	5 units	10 units
5 mg	2 mL	2,500 mcg/mL	4 units	10 units	20 units
5 mg	2.5 mL	2,000 mcg/mL	5 units	12.5 units	25 units
6 mg	3 mL	2,000 mcg/mL	5 units	12.5 units	25 units
10 mg	2 mL	5,000 mcg/mL	2 units	5 units	10 units
10 mg	5 mL	2,000 mcg/mL	5 units	12.5 units	25 units
15 mg	3 mL	5,000 mcg/mL	2 units	5 units	10 units

For any other combination, the peptide reconstitution calculator does the arithmetic and shows the units on a 100, 50 or 30 unit syringe.

Worked example

A pharmacy dispenses sermorelin 6 mg with instructions to add 3 mL of bacteriostatic water. Concentration: 6 × 1,000 ÷ 3 = 2,000 mcg per mL. A prescribed 300 mcg dose is 300 ÷ 2,000 = 0.15 mL, which is 15 units on a U-100 insulin syringe. The vial holds 6,000 ÷ 300 = 20 doses.

Step by step

Wash your hands with soap and water and dry them. Set out everything on a clean surface.
Let the vials reach room temperature. Take the peptide vial out of the refrigerator 15 to 30 minutes before mixing. Check the label: name, strength, expiration date, diluent and volume.
Remove the plastic caps from the peptide vial and the water vial. The rubber stopper underneath is not sterile on the outside.
Swab both stoppers with a fresh alcohol pad and let them air-dry for a few seconds. Wet alcohol carried into the vial can affect the solution; it also stings.
Draw up the water. Pull the plunger of the mixing syringe back to the volume of water you will add, insert the needle into the water vial, inject that air (it keeps the pressure neutral), turn the vial upside down and draw the specified volume. Tap out bubbles and push the plunger to the exact line.
Add the water slowly. Insert the needle through the stopper of the peptide vial at an angle, with the tip against the inside wall of the glass. Push the plunger slowly so the water runs down the wall rather than jetting onto the powder.
Swirl, do not shake. Roll the vial gently between your fingers or swirl it. Most peptides dissolve within one to two minutes; some take longer. Let a stubborn vial sit in the refrigerator for 10 to 15 minutes and swirl again.
Inspect. The solution should be clear, colorless and free of particles. Hold it against a light. Anything else means stop and call the pharmacy.
Label the vial with the date, the concentration you calculated and, if helpful, the units per dose. Write it on tape rather than on the pharmacy label.
Refrigerate at 2 to 8 degrees Celsius, upright, in the original carton or another light-blocking container. Put the used syringe and needle in the sharps container.

How long a reconstituted vial lasts

Two clocks run at the same time.

The beyond-use date on the pharmacy label. This is the date the pharmacy determined for the reconstituted product. It accounts for the preservative and, where data exist, the stability of that specific peptide in solution. It is the one to follow.

The 28-day convention for multi-dose vials. The CDC injection-safety guidance states that once a multi-dose vial is opened or punctured it "should be dated and discarded within 28 days unless the manufacturer states another date for that opened vial," and that the date "should never exceed the manufacturer's original expiration date." The CDC attributes the 28-day figure to USP General Chapter 797. It describes the limit of the preservative, not of the peptide: a peptide can lose potency in solution well before day 28, which is why some pharmacies label shorter periods.

A vial reconstituted with sterile water (no preservative) does not get 28 days. The CDC rule for single-dose containers is that leftover contents are not kept for later use, even for the same person.

Storage, light and travel

Lyophilized vials: refrigerated, 2 to 8 degrees Celsius, in the carton. Brief room-temperature exposure during shipping is normal; days at room temperature are not.
Reconstituted vials: refrigerated, upright, protected from light. Do not freeze; ice crystals destroy the peptide.
Bacteriostatic water: room temperature is fine for the unopened vial; once in use, date it and follow the same 28-day convention.
Travel: an insulated case with a cold pack that does not touch the vial. Take the pharmacy label with you; a labeled prescription vial and syringes are routine at airport security in the United States.
Heat: a car in summer or a windowsill will degrade a peptide in hours. If a vial was left warm for a day, call the pharmacy before using it.

Mistakes that ruin a vial or hurt you

Shaking the vial. Foam, denatured peptide, inaccurate doses.
Using the wrong diluent. Sterile water in a vial you plan to use for weeks; tap or distilled water in anything.
Guessing the volume. Adding "about 2 mL" instead of a measured 2 mL changes every dose that follows.
Not swabbing stoppers, or touching the needle. The two most common routes of contamination.
Reusing needles. Dull needles hurt, tear tissue and carry bacteria into the vial. The CDC rule is a new sterile syringe and needle for every injection.
Drawing from a vial with no date on it. You cannot know whether it is day 5 or day 35.
Freezing, or leaving the vial on the counter.
Mixing peptides at home. Compatibility is untested unless the pharmacy prepared the combination.
Using a gray-market vial. A "research use only" vial has no pharmacy label, no verified content, no sterility testing you can see and no beyond-use date. In August 2026, FDA sent warning letters to five online peptide sellers; the letters treated the presence of reconstitution calculators and bundled bacteriostatic water on those websites as evidence that the vials were intended as injectable drugs for people, and the "research use only" labels did not change that.

Bottom line

Reconstitution is simple arithmetic plus clean technique. Add the labeled volume of the labeled diluent slowly, swirl, confirm the solution is clear, write the date and concentration on the vial, refrigerate, and use a new syringe each time. The beyond-use date on the pharmacy label is the authority on how long the vial lasts; 28 days is the ceiling for a preserved multi-dose vial, not a promise for every peptide.

Frequently asked questions

+Can I use sterile water instead of bacteriostatic water?

Only if your pharmacy label says so, and only for a vial you will use up quickly. Sterile water for injection has no preservative, so a vial that is punctured more than once has no protection against bacterial growth. Bacteriostatic water contains 0.9% benzyl alcohol, which is why a reconstituted multi-dose vial can be kept in the refrigerator and drawn from for up to 28 days.

+How long does a reconstituted peptide last?

Follow the beyond-use date on your pharmacy label. In the absence of a specific date, the USP General Chapter 797 convention that the CDC also cites is 28 days after the first puncture for a multi-dose vial, refrigerated, and never beyond the manufacturer expiration date. Some peptides are less stable than that in solution, which is why the label wins.

+How much bacteriostatic water do I add to a 5 mg vial?

Whatever volume your pharmacy specified. The water volume does not change how much peptide you have; it only sets the concentration. Adding 1 mL to 5 mg gives 5,000 mcg per mL; adding 2 mL gives 2,500 mcg per mL; adding 2.5 mL gives 2,000 mcg per mL. More water means more syringe units per dose, which makes small doses easier to measure accurately.

+Can I use distilled or tap water?

No. Distilled water from a store is not sterile and not pyrogen-free, and tap water is neither. Injecting either carries a real risk of infection and fever reactions. Use only bacteriostatic water for injection or sterile water for injection, as directed on the label.

+What if the solution is cloudy or has particles?

Do not inject it. A correctly reconstituted peptide is clear and colorless. Cloudiness, particles or a color change can mean the peptide degraded, the vial was shaken too hard, or the vial is contaminated. Call the dispensing pharmacy.

+Can I mix two peptides in one vial or one syringe?

Not unless the pharmacy prepared or directed that combination. Compatibility and stability of two peptides in the same solution are compound-specific and usually untested. Some pharmacies dispense a tested combination (for example, two growth hormone secretagogues) in one vial; that is different from mixing at home.

+I added too much water. Is the vial ruined?

No. The peptide is still there; it is just more dilute. Recalculate the concentration (peptide mg times 1,000 divided by total water mL) and redo the dose arithmetic, or use the calculator. The vial is ruined only if it was contaminated, shaken into foam, frozen, or left warm for days.

+Does a reconstituted vial need to be refrigerated?

Yes, 2 to 8 degrees Celsius (36 to 46 Fahrenheit), protected from light and never frozen. Lyophilized powder is also normally stored refrigerated, although many products tolerate short periods at room temperature in transit. Check the label for your product.