Short answer: Bacteriostatic water is sterile water for injection with 0.9 per cent, or 9 mg per ml, benzyl alcohol added as a preservative, and its label describes a multiple-dose container from which repeated withdrawals may be made. Sterile Water for Injection contains no bacteriostat, no antimicrobial agent and no added buffer, and its label says it is intended only for single-dose use, with the unused portion discarded. For a laboratory vial that will be entered more than once, bacteriostatic water is the standard diluent. For a single entry, sterile water is the pharmacopoeial choice.
Disclosure: Peps and Protocols is published by the team behind Vitality Peps. Vendor facts below were checked on each vendor's public website on the date shown, and the criteria are the same for every vendor.
One preservative, and one sentence on each label. Bacteriostatic Water for Injection USP is described on its US label as a sterile, nonpyrogenic preparation of water for injection containing 0.9 per cent, or 9 mg per ml, benzyl alcohol added as a bacteriostatic preservative, supplied in a multiple-dose container from which repeated withdrawals may be made to dilute or dissolve drugs for injection. Its pH is 5.7, within a range of 4.5 to 7.0, and it is stored at 20 to 25 degrees Celsius.
Sterile Water for Injection USP is the opposite by design. Its label states that it is a sterile, nonpyrogenic, solute-free preparation of distilled water, that it contains no bacteriostat, antimicrobial agent or added buffer, and that it is intended only for single-dose injection after admixture with an appropriate solute. When smaller amounts are required, the unused portion should be discarded. Its pH is 5.5, within 5.0 to 7.0.
| Property | Bacteriostatic water | Sterile water for injection |
|---|---|---|
| Composition | Water for injection plus benzyl alcohol 0.9 per cent, 9 mg per ml | Distilled water, solute free, no additives |
| Preservative | Yes, benzyl alcohol | None, no bacteriostat or antimicrobial agent |
| Container per label | Multiple-dose, repeated withdrawals | Single-dose, discard the unused portion |
| pH | 5.7, range 4.5 to 7.0 | 5.5, range 5.0 to 7.0 |
| Label warnings | Not for use in neonates; not for fluid replacement; not for epidural or spinal use | Hypotonic, not to be given without a solute |
| Storage | 20 to 25 degrees Celsius | Room temperature, protect from freezing |
| Typical laboratory use | Vials entered more than once | A single entry, or a preservative sensitive assay |
Chemically, yes: the peptide dissolves either way. Procedurally the two are not interchangeable, because the label difference is about what happens after the stopper is first pierced. Sterile water carries no preservative, so once the container is opened there is nothing suppressing microbial growth in it, which is why the label restricts it to a single dose and tells the user to discard the remainder.
That is the practical reason bacteriostatic water became the default reconstitution solvent in laboratory work with lyophilised peptides. A 5 mg or 10 mg vial is rarely consumed in one withdrawal, and every additional entry is another opportunity for contamination. Our reconstitution guide sets out the aseptic sequence for either diluent.
No. Tap water and household distilled water are neither sterile nor pyrogen free, and there is no specification behind them. Water for injections is a pharmacopoeial grade with its own European Pharmacopoeia monograph, 0169, and the EMA maintains a separate guideline on the quality of water for pharmaceutical use. Ordinary water fails those specifications on microbial count, endotoxin and conductivity.
Physiological saline is a closer call and a common substitution in German forums, where people ask why sodium chloride does not work. Standard 0.9 per cent sodium chloride injection is isotonic but preservative free, so it has the same single-use limitation as sterile water. It is a reasonable single-entry diluent, not a multi-entry one, and it changes the ionic strength of the resulting solution.
Treat this strictly as a solvent volume. In laboratory practice a lyophilised vial of 5 mg or 10 mg is commonly reconstituted with roughly 1 to 3 ml of diluent, chosen so that the resulting stock concentration is convenient to measure on the graduations of the syringe being used. Adding more water does not weaken or damage the peptide, it simply produces a more dilute stock.
Our reconstitution calculator converts a chosen volume and vial mass into a stock concentration. We give no guidance on administration to people, and nothing here is a dose.
In continental Europe, usually not at a pharmacy counter. Bacteriostatic Water for Injection is a licensed prescription product in the United States, marketed under a New Drug Application. In the United Kingdom a bacteriostatic water product does hold a marketing authorisation, held by Pfizer Limited under PL 00057/0934, but its summary of product characteristics lists no therapeutic indication of its own: it is authorised as the diluent for one named alprostadil product, at 9.45 mg per ml benzyl alcohol.
German pharmacies stock the preservative free grade instead. The German medicines database lists Wasser fuer Injektionszwecke under ATC code V07AB01, sold as Aqua ad iniectabilia in ampoules and bottles. That is why German buyers report that bacteriostatic water is unavailable in Apotheken and are offered water for injections instead. Research suppliers therefore sell it as a laboratory reagent rather than as a medicine, including our own 10 ml bacteriostatic water.
This article is general information, not legal advice; rules change and enforcement varies, so check the current position with a qualified adviser in your country.
Two different questions get mixed here: how long the diluent keeps, and how long the reconstituted peptide keeps. Supplier pages very commonly state a 28-day in-use window after the first puncture. That figure is a widely repeated industry convention. It does not appear on the US Bacteriostatic Water for Injection label we checked, which instead tells the user to mix thoroughly, use promptly, and not to store reconstituted solutions unless the manufacturer of the solute directs otherwise.
For the reconstituted peptide, standard laboratory practice is refrigeration, protection from light, and avoiding repeated warming and freeze-thaw cycles. Treat any claimed shelf life as the supplier's claim unless a stability study supports it.
Almost certainly not, but shaking is still worth avoiding. The three mistakes that recur in beginner threads are shaking instead of swirling, warming the diluent to speed dissolution, and reusing a needle between entries. Vigorous shaking creates foam and an air-liquid interface, which is the classic route to protein denaturation and aggregation; slow swirling avoids it.
Warm water is a worse idea, because heat is the main accelerator of peptide degradation, and the label storage range for the diluent is 20 to 25 degrees Celsius. Reusing needles is the mistake with the clearest consequence: the preservative in bacteriostatic water suppresses bacterial growth, it does not sterilise a contaminated needle or a stopper that was not wiped. Aim the diluent stream at the vial wall, let the powder dissolve on its own, and use a fresh sterile needle for each entry.
Standard 0.9 per cent sodium chloride injection is sterile and isotonic but contains no preservative, so it carries the same single-use limitation as sterile water and does not support repeated withdrawals. It also changes the ionic strength of the resulting solution. For a vial that will be entered once, it is a reasonable diluent.
Because of the benzyl alcohol. The US label carries an explicit warning that benzyl alcohol has been associated with toxicity in neonates, and directs that only preservative free sterile water be used where water is required to prepare or dilute medications for that group. The EMA also maintains a scientific guideline on benzyl alcohol as an excipient.
No. The mass of peptide in the vial does not change when the solvent volume changes, so more diluent simply produces a more dilute stock solution and a larger measured volume for the same mass. Choose the volume that gives a concentration you can measure accurately on your syringe graduations, then keep it consistent across a batch.
Better not to. Shaking generates foam and a large air-liquid interface, which is a well known route to protein denaturation and aggregation. Direct the diluent down the inside wall of the vial, then swirl gently and let the powder go into solution by itself. Undissolved material usually means too little time, not a ruined vial.
No. A needle is blunted and contaminated after a single entry, and the preservative in bacteriostatic water suppresses bacterial growth in the solution rather than sterilising anything introduced into it. Use a fresh sterile needle for each withdrawal and wipe the stopper with alcohol beforehand. This is the single cheapest contamination control available.
Mostly from laboratory suppliers rather than pharmacies. German pharmacies dispense preservative free Wasser fuer Injektionszwecke under ATC V07AB01, and the only bacteriostatic water product we could find with a European marketing authorisation is a UK licence limited to use as the diluent for one named medicine, so research suppliers sell it as a reagent.
Product characteristics above are quoted from regulator published labelling. Vitality Peps products are supplied for research use only, and this article gives no administration guidance. Published 3 September 2026.