What bacteriostatic water is
Bacteriostatic water for injection (BWFI) is USP-grade sterile water containing 0.9% benzyl alcohol. The preservative is bacteriostatic, meaning it stops bacteria from multiplying in the vial after the seal is punctured. That single property is what separates it from plain sterile water for injection, which is single-entry and must be discarded after one use. For peptide work — where a 10 mg vial is drawn from over weeks — a bacteriostatic diluent is the practical default.
Bacteriostatic water
Sterile water (SWFI)
Why peptides need it
Lyophilized peptides are stable as a dry cake but must go into solution to be used. The diluent has to be sterile, injection-grade, chemically compatible with the peptide, and — because most peptide vials contain enough material for many doses — able to inhibit microbial growth between draws. Bacteriostatic water meets all four requirements for the majority of therapeutic and research peptides, including GLP-1 agonists (semaglutide, tirzepatide, retatrutide), healing peptides (BPC-157, TB-500), and growth-hormone peptides (ipamorelin, CJC-1295).
Mixing chart for common peptides
The right amount of bacteriostatic water depends on the vial strength and how you want the math to read on the syringe. Concentrations of 2.5 mg/mL and 5 mg/mL are the two most useful landing points for insulin-syringe draws.
| Peptide vial | Bacteriostatic water | Final concentration | Per 0.1 mL draw |
|---|---|---|---|
| Semaglutide 5 mg | 2 mL | 2.5 mg/mL | 250 mcg |
| Semaglutide 10 mg | 2 mL | 5 mg/mL | 500 mcg |
| Semaglutide 10 mg | 1 mL | 10 mg/mL | 1 mg |
| Tirzepatide 10 mg | 2 mL | 5 mg/mL | 500 mcg |
| Tirzepatide 15 mg | 3 mL | 5 mg/mL | 500 mcg |
| Retatrutide 10 mg | 2 mL | 5 mg/mL | 500 mcg |
| BPC-157 5 mg | 2 mL | 2.5 mg/mL | 250 mcg |
| TB-500 5 mg | 2 mL | 2.5 mg/mL | 250 mcg |
| Ipamorelin 5 mg | 2 mL | 2.5 mg/mL | 250 mcg |
| CJC-1295 5 mg | 2 mL | 2.5 mg/mL | 250 mcg |
Rule of thumb: concentration (mg/mL) = vial strength (mg) ÷ diluent volume (mL). Dose per 0.1 mL = concentration ÷ 10.
Correct reconstitution technique
- Bring both vials to room temperature. Cold peptide cake dissolves poorly.
- Wipe both rubber stoppers with a fresh alcohol swab and let dry.
- Draw the target volume of bacteriostatic water into a sterile syringe.
- Insert the needle into the peptide vial at an angle. Aim the stream at the glass wall, not the peptide cake.
- Release the plunger slowly. Do not force water directly onto the powder.
- Roll the vial gently between your palms for 30 to 60 seconds. Do not shake — shearing damages peptide.
- Inspect: solution should be clear and colorless. Cloudiness or particulate means discard.
- Label with peptide, concentration, and reconstitution date. Refrigerate at 2–8°C.
Storage after reconstitution
Refrigerated 2–8°C
Frozen -20°C
Frozen -80°C
See our full peptide reconstitution and storage guide for peptide-specific timelines and freeze-thaw guidance.
Common mistakes
Shaking the vial
Using tap or distilled water
Room temperature storage
Assuming the peptide is what the label says
Sterile diluent handles microbes. Testing handles everything else.
Bacteriostatic water is designed to keep the solution sterile after reconstitution. It does nothing to verify that the powder in the vial matches the label — that requires HPLC purity testing to quantify the target peptide against impurities, mass spectrometry to confirm identity, and, for injectable use, LAL endotoxin testing to rule out pyrogens. A perfectly reconstituted low-purity peptide is still a low-purity peptide.