Bacteriostatic water vs sterile water — why the difference matters for peptides

Bacteriostatic water and sterile water serve different purposes in peptide reconstitution. Understanding this distinction is fundamental to proper peptide handling.

Bacteriostatic water (BAC water):
Sterile water with 0.9% benzyl alcohol added as a preservative. The benzyl alcohol inhibits bacterial growth, which means the water (and anything reconstituted with it) remains safe for multi-dose use. When a peptide vial is reconstituted with BAC water, that vial can be accessed multiple times over 2-4 weeks without significant contamination risk.

This is the appropriate choice for peptide reconstitution in virtually all cases.

Sterile water:
Pure water that has been sterilized but contains no preservative. Once the seal is broken, nothing prevents bacterial growth. Sterile water is intended for single-use applications — open it, use it all, discard it.

If a peptide is reconstituted with sterile water and then drawn from multiple times over days or weeks, bacteria are being introduced with each needle insertion into a solution with zero antimicrobial protection. The peptide will degrade, and contaminated solution may be injected.

When would sterile water be appropriate:
Only if the entire reconstituted vial will be used in a single session. Some researchers use sterile water for compounds that are sensitive to benzyl alcohol (extremely rare in the peptide space). If a specific reason exists to avoid BAC water, sterile water is acceptable for immediate, single-use reconstitution only.

Sourcing BAC water:
It is available from medical supply companies, some pharmacies, and research chemical suppliers. A 30mL vial typically costs $3-8. Do not attempt to make it independently. Do not buy from random sources — ensure it is properly sealed and labeled with lot numbers.

How much BAC water to add:
This depends on the peptide quantity in the vial and the concentration desired. The most common approach is adding enough water to create a convenient concentration for dosing. For example, if a 5mg vial exists and 250mcg per 5 units on an insulin syringe is desired, 1mL of BAC water would be added (giving 5mg/mL, where each 0.05mL = 250mcg).

Using a peptide reconstitution calculator is recommended if the math is unclear. Getting the concentration right is critical for accurate dosing.

Storage after reconstitution:
Reconstituted peptides must be refrigerated immediately. Do not freeze reconstituted peptides. Use BAC water reconstitutions within 2-4 weeks. Discard immediately if the solution becomes cloudy, changes color, or develops visible particles.