Choosing the right water for peptide reconstitution is not a trivial decision. Using the wrong type can lead to bacterial contamination, degraded peptides, or worse — infections. The two options are bacteriostatic water (BAC water) and sterile water for injection. They look identical but have a critical difference.
The Key Difference
| Property | Bacteriostatic Water | Sterile Water for Injection |
|---|---|---|
| Composition | Sterile water + 0.9% benzyl alcohol | Sterile water only |
| Preservative | Yes (benzyl alcohol) | None |
| Multi-dose use | Yes — designed for repeated punctures | No — single use only |
| Shelf life after opening | 28 days | Use immediately, discard remainder |
| Bacterial growth inhibition | Yes | No |
| Cost | Slightly higher | Lower |
| Typical packaging | 30mL multi-dose vial | Various sizes, often single-use ampules |
Why Bacteriostatic Water Is the Standard for Peptides
When you reconstitute a peptide, you will draw from the vial multiple times over days or weeks. Each needle puncture introduces a small risk of bacterial contamination. The 0.9% benzyl alcohol in bacteriostatic water acts as a preservative that inhibits bacterial growth, making the solution safe for repeated access over its 28-day use window.
Without this preservative, bacteria can multiply rapidly in a nutrient-rich solution at refrigerator temperatures. Within 48-72 hours, a contaminated vial can harbor dangerous bacterial colonies that are invisible to the naked eye.
When to Use Sterile Water
Sterile water without preservative is appropriate only when:
- You will use the entire vial in one dose — no repeated punctures needed
- The compound is incompatible with benzyl alcohol — some compounds can be destabilized by preservatives (rare for peptides)
- Benzyl alcohol sensitivity — some individuals are allergic to benzyl alcohol (very uncommon)
- Neonatal use — benzyl alcohol is contraindicated in newborns (not applicable to peptide research)
The Danger of Using Sterile Water for Multi-Dose Vials
This is the most important safety message in this article: never use plain sterile water for a vial you will draw from more than once. Here is what can happen:
- Day 1: You reconstitute with sterile water and draw your first dose. Small amount of bacteria enters from the needle puncture.
- Day 2-3: Bacteria begin multiplying in the solution. The vial looks perfectly clear.
- Day 5-7: Bacterial colony has reached significant levels. You inject contaminated solution.
- Result: Injection site infection, abscess, or systemic infection.
How to Store Bacteriostatic Water
- Unopened: Room temperature, away from light. Shelf life of 2+ years (check expiration date).
- After first puncture: Room temperature is fine, but many people refrigerate for extra safety. Use within 28 days.
- Label it: Write the date you first punctured the vial on the label. Discard after 28 days regardless of how much is left.
What About Normal Saline?
Bacteriostatic sodium chloride 0.9% (bacteriostatic saline) can also be used for peptide reconstitution. It contains the same benzyl alcohol preservative as BAC water plus 0.9% sodium chloride. Some peptides may be more stable in saline than in plain water, but for most peptides, BAC water is the standard and more widely available option.
Plain sterile saline (without benzyl alcohol) has the same limitation as plain sterile water — it is not preserved and should not be used for multi-dose vials.
Quick Decision Guide
| Situation | Use This |
|---|---|
| Reconstituting a multi-dose peptide vial | Bacteriostatic water |
| Single-use, entire vial in one dose | Either (sterile water is fine) |
| Sensitive to benzyl alcohol | Sterile water (single use only) |
| Unsure which to use | Bacteriostatic water (always the safer choice) |
Can I make my own bacteriostatic water?
While it is technically possible to add benzyl alcohol to sterile water, this is not recommended for injection use. Commercially produced bacteriostatic water is manufactured under sterile conditions with precise concentrations. DIY solutions carry significant contamination and concentration risks.
Does bacteriostatic water affect peptide potency?
The 0.9% benzyl alcohol in bacteriostatic water does not meaningfully affect the potency of commonly used research peptides. Some very sensitive proteins may be affected, but standard peptides like BPC-157, Ipamorelin, CJC-1295, and TB-500 are all compatible with BAC water.