What bacteriostatic water is

Bacteriostatic water for injection (BWFI) is sterile water containing 0.9% benzyl alcohol as a preservative. The benzyl alcohol prevents microbial growth in the vial after the first puncture, which is why it is the standard diluent for multi-dose peptide reconstitution in research settings.

What to look for

1. Preservative concentration

0.9% benzyl alcohol is the standard. Some products list 0.9% "with benzyl alcohol as preservative" — that is the same specification. Products without a preservative are sterile water, not bacteriostatic, and support only single-use draws.

2. Sterility documentation

Look for USP-grade sterile production and a documented sterilization method. A COA or lot documentation with a sterility statement should be available.

3. Vial format

30 mL multi-dose vials are the most common lab format. Smaller (10 mL) vials reduce waste for low-volume workflows; 30 mL is the volume that pairs cleanly with typical 10 mg peptide vials.

4. Rubber stopper quality

A halobutyl stopper that reseals cleanly after repeated puncture keeps sterility across the working life of the vial. Cheap latex-adjacent stoppers can core after 5–10 punctures.

5. Compatibility with peptide chemistry

Benzyl alcohol is compatible with the vast majority of research peptides at 0.9%. Two exceptions worth checking against your specific compound:

• Very high-mass or aggregation-prone peptides — some published protocols specify sterile water without preservative to minimize interaction risk.
• Very short peptides in specific formulations — check the source's stated diluent recommendation.

For retatrutide, tirzepatide, semaglutide, BPC-157, TB-500, GHK-Cu, KPV, SS-31, MOTS-c, and NAD+ (typical lab handling), 0.9% BWFI is the standard choice.

Storage

Store BWFI vials at controlled room temperature (20–25°C). After first puncture, the preservative extends usable life to approximately 28 days at 2–8°C for most peptide reconstitutions — verify against the specific peptide's stability data.

What to avoid

• Non-USP sterile water sold without documentation
• Vials without a preservative labeled as "bac water"
• Repackaged BWFI from a syringe rather than a sealed vial
• Vials with visible particulates, discoloration, or a compromised stopper on arrival

Summary

The best bacteriostatic water for peptide research is USP-grade, 0.9% benzyl alcohol, sealed in a 30 mL multi-dose vial with a halobutyl stopper, from a supplier that provides sterility documentation. Everything else is a compromise.

Research use only. Not for human consumption.