Bacteriostatic water vs. sterile water vs. saline
These three get mixed up constantly, so here's the difference in plain terms:
- Bacteriostatic water contains 0.9% benzyl alcohol as a preservative. That's what allows a single vial to be punctured and drawn from repeatedly over an extended period without contamination — the standard choice whenever a reconstituted vial won't be used all at once.
- Sterile water for injection has no preservative. It's intended for a single withdrawal; once punctured, any unused portion should be discarded rather than stored for later use.
- Saline (0.9% sodium chloride) also contains no preservative, and its ionic content can behave differently with certain peptide sequences compared to plain water. It's not a substitute for bacteriostatic water in multi-use reconstitution.
If a vial will be used in one sitting, the choice matters less. If any portion will be stored and drawn from again later, bacteriostatic water is the standard for a reason.
How much bacteriostatic water should I use?
The amount of bacteriostatic water added determines the final concentration of the solution — that's the entire relationship, and we've already broken down the exact math in our Reconstitution Guide, along with the reconstitution technique itself (temperature, technique, and how to spot a problem before it becomes one). Rather than repeat that here, use our Reconstitution Calculator directly — enter your vial size and target concentration and it returns the exact figure, for any vial size we carry.
Frequently asked questions
How much bacteriostatic water do I use per mg of peptide?
It depends entirely on the concentration you're aiming for, not a fixed rule — see our Reconstitution Guide for how the math works, or use the Reconstitution Calculator to get the exact figure for your vial directly.
Can I use sterile water instead of bacteriostatic water?
Only if the entire reconstituted vial will be used immediately in one session. Sterile water has no preservative, so any portion left after the first withdrawal shouldn't be stored for later use.
How long does bacteriostatic water last once opened?
The benzyl alcohol preservative maintains sterility for an extended period after first puncture provided the vial is stored correctly and the stopper is cleaned before each use. See our Storage and Handling guide for full protocols.
Can I reuse the same vial of bacteriostatic water?
Yes — that's the entire purpose of the benzyl alcohol preservative. That's what distinguishes it from sterile water, which is intended for single use only.
This article is provided for general research and educational purposes. It describes laboratory handling procedure only and does not constitute guidance for human use of any compound. All products referenced are sold strictly for laboratory research use.