Bacteriostatic water contains 0.9% benzyl alcohol as a preservative. Sterile water does not. That single difference determines which one you should use and for how long.
Bacteriostatic Water (BAC Water)
Bacteriostatic water is sterile water with 0.9% benzyl alcohol added. The benzyl alcohol inhibits bacterial growth, which means the water stays safe for repeated needle entries over multiple days.
Key properties:
- Contains 0.9% benzyl alcohol
- Inhibits (does not kill) bacterial growth
- Safe for multi-dose vials
- 28-day shelf life after first puncture
- Available in 10ml and 30ml vials
- Requires refrigeration after opening
BAC water is the standard choice for peptide reconstitution. Since most peptide vials are used over days or weeks (drawing one dose at a time), the preservative keeps the solution safe between uses.
Sterile Water for Injection
Sterile water for injection (SWFI) is purified water that has been sterilized and contains no additives. It is free of preservatives, antimicrobial agents, and buffers.
Key properties:
- No preservatives of any kind
- Sterile only until first puncture
- Must be used immediately after opening
- Single-use only
- Available in ampules and vials
Once you puncture a sterile water vial, bacteria from the needle (even a clean one) can enter. Without preservative, those bacteria multiply freely. The solution can become contaminated within hours.
When to Use Which
Use bacteriostatic water when:
- You are reconstituting a multi-dose peptide vial
- You will draw from the vial over multiple days
- You are following a protocol that spans days or weeks (which is nearly all peptide protocols)
This covers the vast majority of peptide use cases. Peptides like BPC-157 (dosed 2x daily for weeks), semaglutide (dosed weekly), and CJC-1295/Ipamorelin (dosed daily) all require BAC water because you return to the same vial repeatedly.
Use sterile water when:
- You will use the entire reconstituted vial in a single session
- The peptide manufacturer specifically requires it
- The individual is sensitive or allergic to benzyl alcohol (rare, but possible)
- Preparing IV formulations (never use BAC water intravenously)
Shelf Life Comparison
| Property | Bacteriostatic Water | Sterile Water | |---|---|---| | Unopened shelf life | 2+ years (check expiry) | 2+ years (check expiry) | | After first puncture | 28 days | Use immediately | | Storage | Room temp unopened, refrigerate after opening | Room temp until use | | Multi-dose safe | Yes | No |
The 28-day window for BAC water aligns well with most reconstituted peptide shelf lives. BPC-157 and semaglutide both last 28 days reconstituted. TB-500 and sermorelin last about 21 days.
Where to Buy
Bacteriostatic water is available from:
- Compounding pharmacies (highest reliability)
- Medical supply websites
- Amazon and general retailers (check for USP grade)
Always look for "USP" on the label. This confirms the water meets United States Pharmacopeia standards for purity and benzyl alcohol concentration.
Sterile water for injection is available from:
- Pharmacies (often behind the counter)
- Medical supply distributors
- Some peptide vendors include it with orders
Buy sterile water in small single-use ampules rather than large vials. This avoids the temptation to reuse an open container.
A Note on Other Liquids
Do not use saline (0.9% sodium chloride) unless the peptide manufacturer specifically calls for it. Some peptides are sensitive to salt concentration. Do not use distilled water from a grocery store. It is not sterile. Do not use any water that is not explicitly labeled for injection use.
The correct choice for nearly every peptide reconstitution scenario is bacteriostatic water. Keep a few 10ml vials on hand, and you will be set for months of use.