Bacteriostatic Water
The default diluent — and the reason a vial can be entered more than once.
Solutions & diluents · 7 min read · updated 13 Aug 2026
Research use only. The ranges below are what published studies and community protocols report. They are reference material for laboratory research, not medical advice or a recommendation for human or veterinary use.
The compound
Bacteriostatic Water
View- Type
- Bacteriostatic Water
- CAS
- 7732-18-5
- Formula
- H₂O / C₇H₈O
- Mass
- N/A
At a glance
- Composition
- Sterile water for injection + 0.9% benzyl alcohol
- CAS (water)
- 7732-18-5
- Function of benzyl alcohol
- Bacteriostatic preservative — permits repeated entry
- pH
- Approximately 5–7
- Sizes stocked
- 3 ml, 10 ml
Bacteriostatic water is sterile water with 0.9% benzyl alcohol added. That single additive is the entire difference between it and plain sterile water, and it changes how the vial can be used: benzyl alcohol inhibits bacterial growth, so a bacteriostatic vial tolerates being entered repeatedly over weeks, while a plain sterile water vial is single-use by definition.
This is why essentially every multi-dose peptide protocol specifies bacteriostatic water. A 10 mg peptide vial reconstituted for twenty doses will be punctured twenty times; without a preservative in the solution, each entry is an opportunity for contamination that then has a warm nutrient-rich medium to grow in.
"Bacteriostatic" means growth-inhibiting, not sterilising. It suppresses proliferation of organisms introduced during handling; it does not disinfect a contaminated vial or make poor technique safe. The stopper still needs an alcohol wipe before every entry.
What benzyl alcohol does and does not do
- It inhibits bacterial replication in the vial over the weeks a reconstituted peptide is in use. This is the reason for the widely used 28-day window — it is a preservative-efficacy interval, not a peptide-stability interval.
- It does not sterilise. An organism introduced in quantity will not be eliminated.
- It has mild local anaesthetic properties, which is why bacteriostatic water stings slightly less on injection than plain sterile water.
- It is not appropriate in neonates — the historical "gasping syndrome" cases are the reason benzyl alcohol carries a specific neonatal warning. Irrelevant to most research contexts but worth knowing why the warning exists.
The two windows people confuse
Reconstituted peptide vials have two separate clocks running, and they are frequently conflated. The first is preservative efficacy — roughly 28 days, set by the benzyl alcohol. The second is peptide stability, which varies by compound: short robust peptides last well beyond a month refrigerated, while recombinant proteins like growth hormone degrade meaningfully within two to three weeks, and glutathione oxidises within days.
The usable life of a vial is whichever window closes first. For most peptides that is the 28-day preservative window; for GH, hCG and glutathione it is the stability window, which is why those articles specify shorter periods.
Technique that actually matters
- Wipe both stoppers — diluent and peptide — with 70% isopropyl alcohol and let them dry. Wet alcohol carried into the vial on a needle does nothing useful.
- Draw the diluent, then inject it against the inside wall of the peptide vial, letting it run down onto the cake. Aiming the stream directly at a lyophilised cake shears the peptide.
- Swirl or roll. Never shake. Shaking creates an air–liquid interface that denatures protein — the damage is invisible and the peptide is simply less active.
- A properly reconstituted vial is clear. Haze, strands or particulates mean it should not be used.
- Use a fresh needle for every entry. Reusing a needle is the single most common source of contamination and of cored stopper fragments.
Choosing a fill volume
The fill volume determines how a dose maps onto syringe units, and getting it wrong makes every subsequent dose an arithmetic problem. The rule that keeps things simple: choose a volume that puts a typical dose between 10 and 50 units on a U-100 insulin syringe. Below 10 units, small measurement errors are proportionally large; above 50, the injection volume becomes uncomfortable.
On a U-100 syringe, 100 units = 1 ml, so 1 unit = 0.01 ml. Concentration in mg/ml divided by 100 gives mg per unit. A 10 mg vial in 2 ml is 5 mg/ml, therefore 0.05 mg per unit.
Storage
- Unopened: room temperature is acceptable; a cool dark cupboard is better in the Malaysian climate.
- Opened: refrigerate at 2–8 °C and treat the same 28-day window as applying.
- Discard if the solution is anything other than clear and colourless.
Cautions
- Benzyl alcohol is not appropriate for neonatal use, and sensitivity to it exists though it is uncommon.
- "Bacteriostatic" is not "sterilising". Technique still carries the load.
- Do not use bacteriostatic water as a large-volume diluent for infusion work — the benzyl alcohol dose scales with volume.
References & credit
- 1USP monograph — Bacteriostatic Water for Injection — The compendial standard defining composition and permitted use.
- 2Gershanik J et al., NEJM 1982 — The neonatal gasping syndrome cases behind the benzyl alcohol warning.
This article was written in-house. Where a dose range reflects community practice rather than a published trial, the community references we consulted are credited above and the article says which is which.