bacteriostatic water

Bacteriostatic Water and Reconstitution Solution: What They Are

Bacteriostatic water is sterile water with a benzyl alcohol preservative. What vendors sell as reconstitution solution, how to read the label, and the legal limits.

Written bySebastian Hunter, Founder and EditorI'm a 42-year-old senior advisor and distance runner who came to peptides as a sceptic. I'm not a doctor: I read the research, run my own experiment and keep honest notes. Read full bio

Published July 16, 2026Updated October 3, 2026

The short version

Bacteriostatic water is sterile water with a preservative, usually benzyl alcohol, so a multi-dose vial can be entered more than once under proper conditions. It differs from plain sterile water, which is why calculators name it specifically. Peptide vendors often sell it as "reconstitution solution", labelled for research use only rather than as a pharmacy product.

It is water plus a bacteriostatic preservative, which makes it different from unpreserved sterile water.
On peptide sites, "reconstitution solution" usually means bacteriostatic water. The ingredients line tells you which liquid you have.
Labels specify strength and limits (including neonate warnings on some products).
I explain maths and product names for education, not reconstitution instructions for human use.

Bacteriostatic water is sterile water with a preservative, benzyl alcohol, so one vial can be entered more than once. Peptide vendors often sell it under the name reconstitution solution. All six products by that name I checked on 3 October 2026 were benzyl alcohol-preserved water labelled for research use only.

What it is

Bacteriostatic Water for Injection, USP is sterile water that contains benzyl alcohol as a bacteriostatic preservative. The preservative is why a multi-dose vial can be entered more than once under appropriate pharmaceutical conditions.

Per Pfizer’s prescribing information for Bacteriostatic Water for Injection, USP:

Benzyl alcohol is present at 0.9% (9 mg/mL) or 1.1% (11 mg/mL) depending on the presentation - so 0.9% is common, not universal.
Approximate pH 5.7 (labelled range 4.5–7.0).
Supplied as multi-dose vials (for example 30 mL presentations described on the label).
Boxed: NOT FOR USE IN NEONATES.

The label also discusses animal-derived estimates of benzyl alcohol exposure. Those estimates do not make unlimited human IV use safe, so read the full label for any product you handle.

Reconstitution solution: what vendors sell under that name

“Reconstitution solution”, or “peptide reconstitution solution”, is a seller’s name for a diluent, the liquid that dissolves a freeze-dried powder. The closest pharmacy product is Bacteriostatic Water for Injection, USP. These are the six products named “Reconstitution Solution” that I checked, in the words of each seller’s page:

Seller	What the page says it contains	Size and price
American Peptides	“a sterile solution with benzyl alcohol”, no strength given	30 mL, $30.00
Aqua Science (bacteriostaticwaters.com)	“99.1% deionized water, 0.9% benzyl alcohol”	2 × 30 mL, $22.99
BioEdge Research Labs	“USP-grade, sterile, bacteriostatic water”, 0.9% benzyl alcohol	30 mL, $29.00 (10 mL, $12.00, out of stock)
Cenexa Labs	“Deionized Pure Water + 0.9% Benzyl Alcohol (USP)”	10 mL, $12.99 (sale; regular $15.99)
G-Biosciences	Sterile water with 0.9% benzyl alcohol; endotoxin ≤0.25 EU/mL	2 × 30 mL, $20.00
Luxe Peptides	Sterile water with 0.9% benzyl alcohol, “USP Grade”	30 mL, $18.00

Prices are as of 3 October 2026, from each seller’s product page. All six are labelled for research use only, and none was plain sterile water. Some sellers also offer 0.6% acetic acid as a separate diluent, and two call it a reconstitution solution too.

How to tell which one you have

Start with the ingredients line on the vial or product page:

Benzyl alcohol, 0.9% (or 1.1% on some pharmacy products): bacteriostatic water, made for a vial entered more than once.
No preservative, single-dose: sterile water. Its label says to discard the unused portion.
Acetic acid (0.6% on the seller pages I checked): an acid diluent, not bacteriostatic water. Pure Peptide Factory’s page gives 0.6% (about 0.1 M) acetic acid in sterile water for injection, at about pH 2.7, with no preservative.

Then check the water and the paperwork:

The Pfizer label describes “a sterile, nonpyrogenic preparation of water for injection”. Cenexa and Aqua Science say deionized water instead. Cenexa’s page names both “Deionized Pure Water” and “Water for Injection, USP”.
A certificate of analysis should match the lot you hold. BioEdge links a Kovera Labs certificate for lot KVR-2026-EF2E86. It reports 0.90% benzyl alcohol, pH 5.7 and “No Growth” in a 2-day rapid sterility screen, and lists no endotoxin result.
Luxe says “CoA on file” but shows no certificate for this product on the page or in its COA library. Aqua Science says a certificate is “available upon request”.
American Peptides lists the form of its liquid as “Lyophilized powder”, which fits a peptide, not a diluent.

My COA guide covers what a certificate should show.

Why the liquid matters

The preservative decides whether a vial can be entered more than once, as the two labels show.

Preservatives don’t suit every molecule. In a lab study, four preservatives, benzyl alcohol among them, made the protein interferon alpha-2a clump, more so at higher concentrations. Benzyl alcohol did this less than m-cresol or phenol Bis 2014 - Bis RL, Mallela KM. Antimicrobial preservatives induce aggregation of interferon alpha-2a: the order in which preservatives induce protein aggregation is independent of the protein. Int J Pharm. 2014;472(1-2):356-61., in the references below. In a Merck lab study, benzyl alcohol did not make a 31-amino-acid peptide aggregate, while m-cresol did Li 2022 - Li M, Falk BT, Lu X et al. Molecular Mechanism of Antimicrobial Excipient-Induced Aggregation in Parenteral Formulations of Peptide Therapeutics. Mol Pharm. 2022;19(9):3267-3278., in the references below.

The right liquid can also be an acid, and lab-protein makers name it product by product. Sartorius’s datasheet lists 10 mM hydrochloric acid for EGF and BDNF, 0.1% acetic acid for interleukin-2, and plain water for its IGF-1 LR3. Pure Peptide Factory markets its acetic acid for peptides that gel in bacteriostatic water, naming AOD-9604 and ARA-290. That is the seller’s claim.

A research-labelled diluent is not a pharmacy product

In the US, Bacteriostatic Water for Injection, USP is labelled “Rx only”. Its label says it is “indicated only for diluting or dissolving drugs for intravenous, intramuscular or subcutaneous injection.”

The research versions are sold without a prescription and labelled for research use only. Aqua Science’s page says its product is not “intended, marketed, promoted, or sold for injection”.

FDA has treated one such product as a drug. Its 24 August 2026 warning letter to Peptide Partners LLC named “Reconstitution Solution (BAC)” as an unapproved new drug. Selling it with injectable peptides, FDA wrote, “demonstrates that you intend for your ‘Reconstitution Solution (BAC)’ to be used in combination for injection.”

The firm’s “for research use only” labelling did not change that. A footnote says that marketing bacteriostatic water to reconstitute the peptides provided “the means to prepare an injectable drug for human administration.” My Peptide Partners review covers the letter.

Not the same as sterile water

Product	Preservative	Typical framing
Sterile Water for Injection	None	Single-use reconstitution contexts
Bacteriostatic Water for Injection	Benzyl alcohol	Multi-dose vial contexts per label

Why peptide calculators mention it

Reconstitution maths needs a diluent volume (mL). Calculators use it to work out concentration and draw volume. They do not turn an unapproved powder into a medicine.

More diluent → lower concentration → larger volume per dose
More diluent does not create more peptide - doses per vial depend on total mass and dose size, not how much water you poured

Worked arithmetic: How to reconstitute peptides · Tools: Dosage calculator · Reconstitution calculator

Sterile water vs bacteriostatic water: what the preservative buys

The two labels differ on one point: whether a mixed vial can be entered again.

Sterile Water for Injection, USP has no preservative. Its DailyMed label says it “contains no bacteriostat, antimicrobial agent or added buffer” and is “supplied only in single-dose containers.” It adds: “Do not reuse single-dose containers. Discard unused portion.” Nothing in it controls microbial growth once the stopper is punctured.

Bacteriostatic Water for Injection, USP adds benzyl alcohol to do that job. That preservative is why the Pfizer label frames the product around multi-dose use, with repeated entries into the same vial.

The difference is about microbial contamination across repeated needle entries. It says nothing about how fast a dissolved peptide breaks down. That depends on temperature, light, concentration and the peptide’s own chemistry, covered in storage and shelf life for reconstituted peptides.

Neither label was written with peptide reconstitution for self-injection in mind. Both are prescribing information for FDA-approved diluents used as a clinician directs.

Legal note for research peptides

If the powder is sold for research use only, reconstitution for human self-administration is outside the approved-drug pathway. FDA looks at intended use, not just a disclaimer sticker. See research peptides and the law.

Printable reconstitution chart

Units per dose for 2, 5 and 10 mg vials at 1, 2 and 3 mL of water, on one page for the fridge. View and print the chart, or have it emailed to you.

Common questions

What is bacteriostatic water?

Bacteriostatic Water for Injection, USP is sterile water that contains benzyl alcohol as a bacteriostatic preservative. The preservative lets a multi-dose vial be entered more than once under appropriate conditions. Product labels specify strength and contraindications.

What is reconstitution solution for?

Reconstitution solution is a liquid for dissolving a freeze-dried (lyophilized) powder, such as a research peptide, so it can be measured out. On peptide and lab-supply sites the name usually means bacteriostatic water, and those versions are labelled for research use only. The pharmacy product, Bacteriostatic Water for Injection, USP, is labelled only for diluting or dissolving drugs for injection.

Is reconstitution solution the same as bacteriostatic water?

Usually. All six products sold as reconstitution solution that I checked on 3 October 2026 were water with benzyl alcohol, and five stated the 0.9% strength. Check the ingredients line: benzyl alcohol means bacteriostatic water, no preservative means sterile water, and acetic acid means an acid diluent. Two research versions say deionized water, where the pharmacy label says water for injection.

Can I buy reconstitution solution over the counter?

Research-labelled reconstitution solution is sold online without a prescription, labelled for research use only. The pharmacy product, Bacteriostatic Water for Injection, USP, is labelled Rx only in the US. FDA's 24 August 2026 warning letter to Peptide Partners LLC treated its "Reconstitution Solution (BAC)", sold alongside injectable peptides, as an unapproved new drug.

Which stores sell bacteriostatic water, such as Walmart or CVS?

The pharmacy product, Bacteriostatic Water for Injection, USP, is labelled Rx only in the US, so a pharmacy needs a prescription to dispense it. I did not check stock at Walmart, CVS or any other individual store. The versions peptide and lab-supply sites sell online are research-labelled, not pharmacy products.

Is acetic acid water a reconstitution solution?

Some sellers offer 0.6% acetic acid in sterile water as a separate diluent, and at least two call it a reconstitution solution. It is acidic (one seller gives a pH of about 2.7) and has no benzyl alcohol, so it is not bacteriostatic water. Makers of lab proteins name the liquid product by product: one datasheet lists water, 10 mM hydrochloric acid or 0.1% acetic acid depending on the protein.

What percentage benzyl alcohol is in bacteriostatic water?

Prescribing information for Bacteriostatic Water for Injection describes 0.9% (9 mg/mL) or 1.1% (11 mg/mL) benzyl alcohol, depending on the product. 0.9% is common but not the only strength.

Is bacteriostatic water the same as sterile water?

No. Sterile water for injection has no preservative. Bacteriostatic water includes a bacteriostatic agent. They are different products with different labelled uses and limits.

Can I use bacteriostatic water to reconstitute peptides?

This site explains reconstitution arithmetic for education. Product selection, sterile technique and whether any compound should be reconstituted for human use are clinical and legal questions. I do not answer them as instructions. Research-use peptides are not approved for human use.

Does bacteriostatic water make a reconstituted peptide last longer than sterile water?

Their labels answer different questions. Sterile Water for Injection, USP contains no preservative. Its label is limited to single-dose use, with instructions to discard any unused portion rather than re-enter the vial. Bacteriostatic Water for Injection, USP contains a benzyl alcohol preservative so that a multi-dose vial can be entered more than once. That difference is about controlling microbial contamination across repeated entries. It says nothing about how fast the peptide itself degrades chemically, which is a separate question covered in my storage and shelf-life guide.

Can I buy bacteriostatic water over the counter, or is a prescription needed?

In the US its label says prescription. Pfizer's label for Bacteriostatic Water for Injection, USP is marked "Rx only", and the DailyMed listing for plain Sterile Water for Injection classes it as a human prescription drug. Both labels say the product is indicated only for diluting or dissolving drugs for injection, according to the instructions of the manufacturer of the drug being given.

What is the best reconstitution fluid for peptides?

I do not rank diluents or give reconstitution instructions for research peptides, which are not approved for human use. What the labels show is the difference that matters. Bacteriostatic water contains a benzyl alcohol preservative so a multi-dose vial can be entered more than once. Sterile water has none, and its label limits it to single-dose use. For an approved drug, the right diluent is the one its manufacturer's instructions name. Lab-reagent datasheets do the same, and some name an acid rather than water.

References (17 sources)

Bibliographic detail is fetched from PubMed, not written by me - so a citation here cannot drift from the paper it names. Study design comes from PubMed's own tags rather than my judgement.