Bacteriostatic water is sterile water with 0.9% benzyl alcohol added, which inhibits bacterial growth. Water for injection is also sterile but contains no such agent. There is one practical difference: a solution prepared with bacteriostatic water keeps in the fridge for about four weeks, while one prepared with water for injection has to be used far sooner.
This article describes the three types of water used when dissolving peptides and explains when each is suitable. The dissolving procedure itself is covered step by step in the article on reconstituting a peptide.
What is bacteriostatic water?
Bacteriostatic water is sterile water with 0.9% benzyl alcohol added. The benzyl alcohol works as a preservative that inhibits the multiplication of microorganisms in the solution. Thanks to it, a finished peptide solution lasts weeks rather than days.
The labelling for Bacteriostatic Water for Injection gives concrete numbers. The preservative concentration is 0.9%, that is 9 mg per millilitre, and in some preparations 1.1%, that is 11 mg per millilitre. The pH is 5.7, within a range of 4.5 to 7.0. The manufacturer describes the vial as a multiple-dose container from which liquid is withdrawn repeatedly. The label limits the role of this water to that of a diluent and solvent.
How does water for injection differ?
Water for injection is sterile but contains no bacteriostatic agent. It is suitable for dissolving a peptide, except that the resulting solution must be used far sooner, because nothing holds back bacterial growth in it. It is a choice for situations where the solution will be used immediately.
Its labelling describes a single-dose preparation without preservative. The second difference often matters more than shelf life: this water is hypotonic. The label requires the solution to be made approximately isotonic before use, because water alone, without a dissolved substance, ruptures red blood cells. Water for injection is therefore a carrier for a substance, not a preparation in itself.
And distilled water?
Distilled water is free of minerals but is neither sterile nor bacteriostatic. It is not suitable for dissolving peptides. The lack of sterility means a risk of introducing contamination at the very stage of preparing the solution.
How long can one vial of water be punctured?
A preserved multiple-dose vial has a default limit of 28 days from first puncture. That is how the United States Centers for Disease Control and Prevention and chapter 797 of the USP pharmacopoeia count it. The limit shortens if the manufacturer states a shorter one, and it never extends beyond the expiry date on the label.
A preservative does not turn a vial into an eternal container. It inhibits bacterial multiplication but does not remove bacteria and does not undo a dirty puncture. Wipe the stopper with an alcohol swab before every withdrawal, not only the first.
Who is benzyl alcohol a problem for?
The label of bacteriostatic water carries a warning in capitals: the preparation is not intended for neonates. The reason is the toxicity of benzyl alcohol itself in that age group, not any property of the water.
For an adult the thresholds look different. The documentation cites studies in several animal species indicating that an intravenous dose of up to 30 millilitres of a preparation with 0.9% benzyl alcohol produced no toxic effects in an adult. That is a limit for the volume of liquid, not a recommendation about any peptide.
Which type of water should you choose?
| Type of water | Sterile | Inhibits bacteria | Solution life in the fridge |
|---|---|---|---|
| Bacteriostatic water | yes | yes (0.9% benzyl alcohol) | about 4 weeks |
| Water for injection | yes | no | short, use quickly |
| Distilled water | no | no | not suitable |
In most situations the choice falls on bacteriostatic water, because it gives the longest life to the finished solution. Water for injection is an option only when the solution will be used straight away.
How much water goes into a peptide vial?
The amount of water does not change the number of milligrams in the vial, only the concentration of the finished solution. A vial with 10 mg of peptide and 2 millilitres of water gives 5 mg per millilitre. The same vial with 1 millilitre gives 10 mg per millilitre, and with 5 millilitres, 2 mg per millilitre.
More liquid makes it easier to measure a small portion, because a syringe graduation then corresponds to fewer milligrams. Less liquid means less space taken up in the vial and a shorter time to use up the whole of it. This article gives no doses or administration schedules, because the peptides in our catalogue have the status of research material.
The most common mistakes in choosing water
- Using distilled or tap water instead of a sterile preparation.
- Treating bacteriostatic water as something that will sterilise an already contaminated solution.
- Puncturing the same water vial for months without tracking the 28-day limit.
- Skipping disinfection of the stopper on subsequent withdrawals.
- Keeping a solution made with plain sterile water as long as one made with a preservative.
Frequently asked questions
How does bacteriostatic water differ from water for injection?
Both are sterile, but only bacteriostatic water contains benzyl alcohol, which inhibits bacterial growth. That is why a solution made with bacteriostatic water lasts about four weeks in the fridge and one made with water for injection far less.
Will a peptide dissolve in water for injection?
Yes, it will. The problem is not dissolution but storage: without a bacteriostatic agent the solution has to be used quickly.
Can distilled water be used?
Distilled water is not used. It is neither sterile nor bacteriostatic, so it is not suitable for preparing a peptide solution.
How long can an opened vial of bacteriostatic water be used?
The default is 28 days from first puncture, following CDC guidance and chapter 797 of the USP pharmacopoeia. A shorter limit stated by the manufacturer and the expiry date on the label take precedence.
Why is bacteriostatic water not intended for neonates?
Because of benzyl alcohol, which is toxic to neonates. The label of the preparation carries an explicit warning against use in that group.
How much water gives a known concentration?
Concentration is the number of milligrams of peptide divided by the number of millilitres of water. A 10 mg vial with 2 millilitres gives 5 mg per millilitre, and with 5 millilitres, 2 mg per millilitre.
Where this information comes from
The composition of bacteriostatic water, the distinction between the three types and the storage times for the solution come from material published by Molequa and PurePoint, checked on 31 August 2026.
The concentrations of 9 mg and 11 mg of benzyl alcohol per millilitre, the pH of 5.7 within a range of 4.5 to 7.0, the multiple-dose container description, the neonate warning and the data on an intravenous dose of up to 30 millilitres in an adult come from the labelling of Bacteriostatic Water for Injection by Hospira, version of August 2019, published in the DailyMed database of the United States National Library of Medicine. The requirement to make water for injection approximately isotonic, and the note on red blood cell rupture without a dissolved substance, come from the same source. The 28-day limit from first puncture of a multiple-dose vial comes from guidance of the United States Centers for Disease Control and Prevention and from chapter 797 of the USP pharmacopoeia. These pages were checked on 7 September 2026.
Sources
- Bacteriostatic Water for Injection, USP (Hospira). Product information in the DailyMed database, U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=87d6e9dc-fe3b-4593-ac9a-d7493d1959c7
- Preventing Unsafe Injection Practices. Centers for Disease Control and Prevention (CDC). https://www.cdc.gov/injection-safety/hcp/clinical-safety/index.html