What is bacteriostatic water, and why do researchers use it for peptides?
Bacteriostatic water — usually shortened to bac water — is sterile water for injection containing 0.9% benzyl alcohol. That small amount of preservative inhibits bacterial growth after the vial has been punctured, which is what makes bac water the default diluent for multi-draw research vials.
Plain sterile water is sterile only until the first puncture. For a vial that will be drawn from repeatedly over days or weeks, bacteriostatic water for peptides keeps the solution usable for roughly 28 days under refrigeration, while sterile water should be treated as single-session.
Bac water vs sterile water vs saline
All three are legitimate diluents; the right choice depends on how many times the vial will be accessed and whether the specific peptide has solubility constraints.
| Diluent | Key property | Best for | Handling notes |
|---|
| Bacteriostatic water (bac water) | 0.9% benzyl alcohol inhibits bacterial growth | Multi-draw research vials used over days or weeks | Refrigerated; typically usable for ~28 days after first puncture |
| Sterile water for injection | No preservative; sterile but single-use in practice | One-time reconstitutions consumed immediately | No preservative — use promptly, discard leftovers |
| Normal saline (0.9% NaCl) | Isotonic salt solution, no preservative | Protocols that specify saline; some peptides are less soluble in saline | Single-use; discard after the session |
How much bac water should you add per vial?
The diluent volume sets the final concentration — and therefore how much solution each draw requires. More bac water means a more dilute solution and larger draw volumes; less means a concentrated solution and smaller, harder-to-measure draws. Common research configurations:
| Vial size | Bac water added | Resulting concentration |
|---|
| 5 mg | 1 mL | 5 mg/mL (5,000 mcg/mL) |
| 5 mg | 2 mL | 2.5 mg/mL (2,500 mcg/mL) |
| 10 mg | 2 mL | 5 mg/mL (5,000 mcg/mL) |
| 10 mg | 3 mL | ≈3.33 mg/mL (3,333 mcg/mL) |
| 15 mg | 3 mL | 5 mg/mL (5,000 mcg/mL) |
| 30 mg | 3 mL | 10 mg/mL (10,000 mcg/mL) |
Skip the mental math: our peptide calculator converts any vial size and bac water volume into concentration, mcg per 0.1 mL, exact draw volume, and U-100 syringe units, and our article peptide concentration math, explained walks through the formulas.
Step-by-step: reconstituting a peptide vial with bac water
- 1. Let the vial reach room temperature. Remove the lyophilized vial from the freezer or refrigerator and let it equilibrate to room temperature for 10–20 minutes while still sealed. Opening a cold vial invites condensation, and moisture is the enemy of peptide stability.
- 2. Sanitize both stoppers. Wipe the peptide vial stopper and the bacteriostatic water vial stopper with separate isopropyl alcohol swabs and let them air-dry.
- 3. Draw the bac water. Pull your planned diluent volume into a sterile syringe. Equalize pressure first by injecting the same volume of air into the bac water vial if it resists the draw.
- 4. Add diluent against the vial wall. Insert the needle at a shallow angle and release the bac water slowly so it runs down the inside glass wall — never spray directly onto the lyophilized cake, which can shear fragile peptide chains.
- 5. Swirl, never shake. Gently roll and swirl the vial until the solution is fully clear. Shaking or vigorous agitation causes foaming and mechanical degradation. Most Opa Peptides vials dissolve within a minute or two.
- 6. Inspect the solution. The finished solution should be perfectly clear and free of particles. Cloudiness, floaters, or discoloration mean the vial should be discarded and documented — not used.
- 7. Label and refrigerate. Mark the vial with the reconstitution date, diluent, volume, and resulting concentration (mg/mL). Store at 2–8°C, upright, protected from light.
For the longer-form version with photographs of each stage, see how to reconstitute a lyophilized research peptide vial.
Storage rules: before and after reconstitution
Lyophilized peptides are remarkably stable when frozen and dry; reconstituted peptides are not. The moment bac water enters the vial, the clock starts — which is why many researchers reconstitute only what a study window requires.
| State | Conditions | Typical window |
|---|
| Lyophilized (unopened) | −20°C freezer, dry and dark | 12–24 months |
| Lyophilized (short term) | 2–8°C refrigerator | Several weeks to months |
| Reconstituted in bac water | 2–8°C refrigerator, upright, away from light | Up to ~28 days |
| Reconstituted in sterile water | 2–8°C refrigerator | Use promptly; days, not weeks |
| Any peptide | Room temperature or repeated freeze–thaw | Avoid — accelerated degradation |
The full rationale — freeze–thaw damage, light exposure, and cold-chain handling from shipment to bench — is covered in storage and cold chain: keeping a research peptide intact, and our shipping standards are documented on the quality standards page.
Common questions
Do you have to use bacteriostatic water for peptides?
No — sterile water and normal saline are valid diluents. Bac water is simply the standard for any vial that will be punctured more than once, because the benzyl alcohol preservative keeps the solution bacteriostatic between draws.
How long does a reconstituted peptide last in the fridge?
With bacteriostatic water at 2–8°C, a typical working window is up to ~28 days. Peptide-specific stability varies — when in doubt, reconstitute smaller batches more frequently.
Can you freeze a reconstituted peptide?
Generally avoid it. Freeze–thaw cycles stress peptide structure. Frozen storage is for the lyophilized powder; once reconstituted, keep the vial refrigerated and upright.
Vials ready for reconstitution
Every Opa Peptides vial ships lyophilized with a batch-specific certificate of analysis. Calculate your diluent volume with the peptide calculator, then order:
For laboratory research use only. Not for human or veterinary use. This guide describes laboratory handling of research materials and is not medical advice.