BAC Water & Peptide Reconstitution Guide
Most peptides — outside of pre-filled prescription pens like Ozempic or Mounjaro — are supplied as a lyophilized (freeze-dried) powder that has to be mixed with a liquid before it can be injected. This guide covers what bacteriostatic water is, how to work out dilution and dosing, and how to handle and store a reconstituted vial.
What Is Bacteriostatic Water?
Bacteriostatic water is sterile water with 0.9% benzyl alcohol added as a preservative. That small amount of benzyl alcohol inhibits bacterial growth in the vial once it's been punctured, which is why it's the standard choice for reconstituting multi-dose peptide vials — it lets a single vial be used for repeated injections over several weeks, rather than requiring a fresh, single-use vial every time the way plain sterile water without a preservative would.
Dilution & Dosing Calculator
Enter your vial size, how much bacteriostatic water you're adding, and your target dose. This does the concentration math for you — it doesn't tell you what dose to take.
Concentration
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Volume to draw
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Insulin syringe units (U-100)
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"Units" refers to a standard U-100 insulin syringe (100 units = 1 mL). Always double-check your syringe's markings match this before drawing a dose.
Step-by-Step Reconstitution
- Gather sterile supplies. A new, sealed vial of bacteriostatic water, alcohol swabs, and a new sterile syringe. Don't reuse needles or syringes between vials.
- Disinfect both vial tops. Wipe the rubber stopper of both vials with an alcohol swab and let them air dry.
- Draw the bacteriostatic water. Draw air into the syringe equal to the volume you plan to withdraw, inject that air into the BAC water vial to equalize pressure, then draw out your intended amount.
- Add it to the peptide vial slowly. Insert the needle at an angle and let the water run gently down the inside wall of the vial rather than injecting it directly onto the powder.
- Mix without shaking. Gently swirl or roll the vial between your palms until fully dissolved. Vigorous shaking can damage the peptide's structure.
- Label and store. Note the reconstitution date and concentration on the vial, then store it refrigerated and protected from light.
Storage & Handling
- Store reconstituted vials refrigerated (not frozen) and away from direct light.
- Label every vial with the date it was mixed — it's easy to lose track once you have more than one going.
- Use a new sterile needle and syringe for every draw to reduce contamination risk.
- Never inject a solution that looks cloudy, discolored, or has visible particles — discard it.
- Dispose of used needles in a proper sharps container, not household trash.
Frequently Asked Questions
What is bacteriostatic water?+
Bacteriostatic water (often shortened to "BAC water") is sterile water with 0.9% benzyl alcohol added as a preservative. The benzyl alcohol inhibits bacterial growth, which is what allows a reconstituted vial to be used multiple times over several weeks instead of having to be discarded after a single use, the way plain sterile water would need to be.
Can I use plain sterile water instead of bacteriostatic water?+
Sterile water without a preservative is generally used for single, immediate-use reconstitution only, since it offers no protection against bacterial growth once the vial is punctured. Most people reconstituting a multi-dose peptide vial use bacteriostatic water specifically so the vial remains usable for its typical refrigerated shelf life.
How much BAC water should I add?+
There's no single correct amount — it depends on what concentration (and therefore what injection volume) you want to end up with. Adding more BAC water gives you a more dilute solution, which means a larger injection volume for the same dose; adding less gives a more concentrated solution and a smaller injection volume. Use the calculator above to work out the specific numbers for your vial.
How long does a reconstituted peptide vial last?+
This varies by peptide and isn't something we can give a universal answer for. As a general reference point, bacteriostatic water itself is typically labeled for use within 28 days of first puncturing the vial when refrigerated, and most reconstituted peptides are stored refrigerated and used within a similar multi-week window. Some peptides degrade faster than others, so check specific guidance for the compound you're using and store any reconstituted vial refrigerated and away from light.
Do I need to refrigerate reconstituted peptides?+
Yes, in almost all cases. Refrigeration (not freezing) slows the breakdown of the peptide once it's in liquid form. Freeze-dried, unreconstituted peptide powder is generally more stable and is often stored refrigerated or frozen until you're ready to mix it.