Bacteriostatic Water for Peptides: How Much to Use, Storage and Shelf Life

Updated September 2026. For laboratory and research use only.

Quick Answer: What Is BAC Water?

Bacteriostatic water — BAC water — is sterile water with 0.9% benzyl alcohol added as a preservative. The preservative is what makes it the standard diluent for lyophilized research peptides: once a sealed vial of BAC water has been entered, it can be drawn from again under sterile technique, which plain sterile water cannot support. The volume of BAC water you add to a peptide vial sets the concentration of the reconstituted solution, and that single fact is behind nearly every question people ask about it.

This guide covers what BAC water is used for, how much to use and why, how it compares with sterile water, “reconstitution solution” and acetic acid water, whether it needs refrigerating, how long it lasts, and the handling mistakes that actually compromise a sample. If you are looking to buy it, USP-grade BAC water in 10mL and 30mL vials is sold on the product page, and one 10mL vial ships free with every peptide order.

What Is Bacteriostatic Water Used For?

In peptide research, BAC water has one job: turning a lyophilized (freeze-dried) peptide back into a solution at a known concentration. Peptides are shipped as a dry powder because the dry state is far more stable in transit and storage than a solution. Before the material can be measured, characterized or used in a protocol, it has to be dissolved — reconstituted — and the diluent chosen for that step determines how the prepared vial can be handled afterwards.

Because most research peptides are supplied in multi-milligram vials that are drawn from repeatedly over days or weeks, a preserved diluent is the practical choice. Bacteriostatic water is also used for:

• Preparing working dilutions from a stock solution when a method calls for a lower concentration.
• Rinsing and priming laboratory syringes and transfer equipment where a sterile, preserved liquid is required.
• Multi-use vial protocols where a single prepared vial supplies several measurements over time.

“Bacteriostatic” means the preservative inhibits bacterial growth; it does not sterilise a contaminated solution. Sterile technique — a clean work surface, an alcohol-wiped stopper, a fresh sterile needle for every entry — is still required every time the vial is accessed.

How BAC Water Changes Peptide Concentration

Every “how much bacteriostatic water” question comes back to one relationship:

Concentration (mg/mL) = peptide mass in the vial (mg) ÷ volume of BAC water added (mL)

More water gives a lower concentration per millilitre; less water gives a higher one. The peptide mass is fixed by the vial you bought. The volume is the only variable you control, so the right amount of BAC water is whatever produces the concentration your method needs — there is no universal number. Two consequences follow:

1. The same vial can be reconstituted to very different concentrations. A 10mg vial with 1mL of BAC water is 10mg/mL; with 2mL it is 5mg/mL; with 4mL it is 2.5mg/mL. All three are correct — for different protocols.
2. Concentration determines how much volume you draw for a given mass. Higher concentration means smaller draw volumes, which are harder to measure precisely on a small syringe; lower concentration means larger draws that are easier to measure but use more of the vial’s capacity.

Worked Examples: Volume Sets Concentration

The table shows how the same relationship plays out across common research peptide vial sizes. These are illustrations of the arithmetic, not protocols or dosing guidance.

Research peptide	Vial mass	BAC water added	Resulting concentration
BPC-157	5mg	2mL	2.5mg/mL
Tirzepatide	5mg	1mL	5mg/mL
Retatrutide	10mg	2mL	5mg/mL
Semaglutide	5mg	2mL	2.5mg/mL
Cagrilintide	5mg	1mL	5mg/mL
GHK-Cu	50mg	5mL	10mg/mL
KLOW blend	80mg	4mL	20mg/mL

Notice that the semaglutide and BPC-157 rows land on the same concentration from the same inputs, and that doubling the water on the retatrutide row would halve it to 2.5mg/mL. The peptide’s identity does not enter the arithmetic at all — only its mass and the volume.

Concentration Formula and Calculator

To work backwards from a target concentration to the volume of BAC water to add:

Volume to add (mL) = peptide mass (mg) ÷ target concentration (mg/mL)

For a 10mg vial and a target of 4mg/mL, that is 10 ÷ 4 = 2.5mL. The MyGLP1Store Reconstitution Calculator does this in both directions and also shows the draw volume for a given mass at the resulting concentration. Calculator outputs are concentration references for controlled research and are not dosing instructions.

Two practical constraints sit on top of the arithmetic:

• Vial headspace. Most lyophilized peptide vials are 2mL or 3mL glass. Adding more BAC water than the vial can hold is not an option, which is one reason higher concentrations are sometimes chosen.
• Syringe resolution. A 1mL syringe marked in 0.01mL steps cannot meaningfully measure a draw below a few hundredths of a millilitre. If the concentration is so high that the required draw is tiny, a lower concentration — more water — gives a measurable volume.

For a step-by-step reconstitution procedure, including how to add the water without foaming the peptide, see the GLP-1 peptide reconstitution guide.

BAC Water vs Sterile Water

This is the comparison that causes the most confusion, because the two liquids look identical and both are sterile. The difference is the preservative, and it changes how the prepared vial can be handled.

	Bacteriostatic (BAC) water	Sterile water
Composition	Sterile water + 0.9% benzyl alcohol	Sterile water, no additives
Preservative	Yes — inhibits bacterial growth after the vial is entered	None
Multi-use vial	Yes, per label and laboratory procedure	No — intended for single entry, discard the remainder
Typical research use	Reconstituting lyophilized peptides that will be drawn from more than once	One-time dilutions; methods where benzyl alcohol must be avoided
Container	Multi-dose vial	Single-dose vial or ampoule

Sterile water is the right choice only when a method specifically requires the absence of benzyl alcohol, or when the entire prepared volume will be used at once. For any peptide vial that will sit in the laboratory and be accessed over days, sterile water leaves the solution unprotected between entries. The two are not interchangeable, and substituting sterile water because it is what was on hand is one of the most common ways a multi-use vial ends up contaminated.

BAC Water vs “Reconstitution Solution”

Some suppliers sell a product labeled “reconstitution solution” or “peptide reconstitution water.” In most cases this is bacteriostatic water under a different name — sterile water with 0.9% benzyl alcohol — but the label is not a guarantee. The way to tell is to read the composition on the vial or the accompanying documentation:

• If it lists water and 0.9% benzyl alcohol, it is bacteriostatic water and behaves exactly as described in this guide.
• If it lists acetic acid, it is an acidified solvent (see the next section) and is not a general-purpose replacement for BAC water.
• If it lists saline (0.9% sodium chloride) with no preservative, it is normal saline: single-entry only, and isotonic rather than preserved.
• If the composition is not stated, do not assume it is bacteriostatic water. A solution with no declared preservative should be treated as single-entry.

The product name tells you what the seller calls it. The composition tells you what it is.

BAC Water vs 0.6% Acetic Acid Water

A minority of research peptides do not dissolve cleanly in neutral water. Compounds such as GHK-Cu and AOD-9604 can show poor solubility or aggregation at neutral pH, and their documentation may specify an acidified solvent — typically 0.6% acetic acid in sterile water. This is a different product with a different purpose.

	Bacteriostatic (BAC) water	0.6% acetic acid water
Main distinction	Preserved, neutral aqueous diluent	Acidified aqueous solvent for low-pH protocols
Preservative	0.9% benzyl alcohol	None
Multi-use vial	Yes, per label	No — no preservative
When it is used	The default for most lyophilized peptides	Only when the compound documentation specifies acidic conditions
Universal substitute?	No	No

Neither solvent replaces the other. Using acetic acid water on a peptide that does not need it changes the pH of the solution for no reason; using BAC water on a peptide whose documentation calls for acid can leave it undissolved. Follow the documentation for the specific compound. The full treatment of which peptides need which solvent is in the acetic acid water vs BAC water guide, and 0.6% acetic acid water is sold separately in 3mL and 10mL bottles.

Does BAC Water Need Refrigeration?

Unopened bacteriostatic water does not need to be refrigerated. It is stored at controlled room temperature — around 20–25°C (68–77°F) — away from direct light and heat, exactly as the label states. Refrigerating a sealed vial does no harm, but it is not required and does not extend the shelf life printed on the label.

The refrigeration question usually arises because of the peptide, not the water. Once a peptide has been reconstituted with BAC water, the resulting solution is a different material with its own stability profile, and most reconstituted peptide solutions are kept refrigerated per the compound’s documentation. Two separate rules, then:

• The BAC water vial: room temperature, sealed, out of the light. After first entry, follow the label and your laboratory procedure.
• The reconstituted peptide: follow the storage guidance for that specific compound, which is usually refrigerated and protected from light.

How Long Is BAC Water Good For?

There are two different answers, and both matter.

Sealed, unopened: the expiry date printed on the vial. USP-grade bacteriostatic water typically carries a shelf life of two years or more from manufacture when stored as directed. Do not use a vial past its printed expiry.

After first entry: the preservative limits, but does not eliminate, the risk of contamination from repeated entries. Manufacturer labeling for USP bacteriostatic water commonly instructs that the vial be discarded a set number of days after it is first punctured — 28 days is the figure that appears on most labels — and that instruction, on the vial in your hand, is the one to follow. Mark the date of first entry on the vial so the window is not guesswork. If at any point the solution is cloudy, discolored or contains visible particles, discard it regardless of date.

Note that these windows apply to the BAC water itself. The reconstituted peptide solution has its own stability window, set by the peptide, and it is usually shorter.

Common Research-Handling Mistakes

1. Using sterile water for a multi-use vial. The solution has no protection between entries. If the vial will be drawn from more than once, it needs a preserved diluent.
2. Adding a fixed “standard” volume without calculating. There is no standard volume. The volume is chosen to produce a concentration, and the concentration is chosen to suit the method and the syringe. Calculate it.
3. Squirting the water directly onto the powder. A forceful stream can foam or denature a peptide. Run the water slowly down the inside wall of the vial and let it dissolve the powder gently; swirl, don’t shake.
4. Entering the vial without wiping the stopper. The preservative slows bacterial growth; it does not sterilise what a dirty needle carries in. Alcohol-wipe the stopper every time.
5. Re-using a needle between the BAC water vial and the peptide vial. Each vial gets a fresh sterile needle. Cross-contamination between vials is one of the most common ways a whole batch of prepared material is lost.
6. Not dating the vial at first entry. The after-opening window on the label is meaningless if nobody knows when the clock started.
7. Treating “reconstitution solution” as interchangeable with BAC water. Read the composition. If it is not water plus 0.9% benzyl alcohol, it is a different product.
8. Using BAC water for a peptide whose documentation specifies acetic acid. The compound may not dissolve. Check the documentation before choosing the solvent.
9. Storing reconstituted peptide at room temperature because the BAC water was. The water’s storage rule does not transfer to the solution made with it.

How to Read the Product Label

The label is the document that tells you what is in the vial and how to handle it. It should state the product name, the composition (water and 0.9% benzyl alcohol), the grade (USP), the vial volume, the lot number, the expiry date, storage conditions and the after-opening instruction. If the composition or the lot number is missing, you cannot verify what you have — and a seller who cannot show you a proper label is telling you something too.

Buying Bacteriostatic Water for Peptide Research

Retail pharmacies do not generally stock bacteriostatic water for laboratory use; it is sold by laboratory and research suppliers. MyGLP1Store sells USP-grade BAC water in sealed 10mL and 30mL vials, as single vials or 3- and 10-vial packs of 10mL, shipped from the United States. One 10mL vial is included free with every peptide vial order, so for most research orders the water arrives in the same box as the peptide.

Frequently Asked Questions

How much bacteriostatic water do I mix with 5mg of semaglutide?

There is no fixed number — the volume sets the concentration. Adding 1mL to a 5mg vial gives 5mg/mL; adding 2mL gives 2.5mg/mL. Choose the volume that produces the concentration your method calls for. This is reconstitution arithmetic for laboratory research, not a dosing recommendation.

How much bacteriostatic water do I mix with 10mg of tirzepatide?

Same principle: 1mL gives 10mg/mL, 2mL gives 5mg/mL, 4mL gives 2.5mg/mL. Use the reconstitution calculator to work from a target concentration to a volume.

How much bacteriostatic water for 10mg of BPC-157?

2mL gives 5mg/mL and 4mL gives 2.5mg/mL. BPC-157 is often reconstituted at a lower concentration than the GLP-1 peptides because the draw volumes involved are smaller, but the arithmetic is identical.

Is bacteriostatic water the same as sterile water?

No. Bacteriostatic water contains 0.9% benzyl alcohol as a preservative and can be drawn from repeatedly under sterile technique. Sterile water has no preservative and is for single entry only.

Is reconstitution solution the same as bacteriostatic water?

Often, but not always. Read the composition on the label. If it is water and 0.9% benzyl alcohol, it is bacteriostatic water. If it lists acetic acid or no preservative, it is something else.

Does bacteriostatic water need to be refrigerated?

No. Store sealed vials at controlled room temperature away from light. Refrigerate the reconstituted peptide according to that compound’s documentation.

How long is bacteriostatic water good for after opening?

Follow the after-opening instruction on your vial’s label; most USP bacteriostatic water labels specify 28 days after first entry. Mark the date of first entry on the vial and discard on schedule, or sooner if the solution changes appearance.

Does bacteriostatic water go bad?

Sealed vials carry a printed expiry date and should not be used past it. Opened vials should be discarded per the label’s after-opening window. Any vial with cloudiness, discoloration or particles should be discarded regardless of date.

Can I use bacteriostatic water for every peptide?

For most lyophilized peptides, yes — it is the default diluent. A small number of compounds specify an acidified solvent instead; check the compound’s documentation, and see the acetic acid water vs BAC water guide.

What is the best type of BAC water for peptides?

USP-grade bacteriostatic water — sterile, non-pyrogenic, 0.9% benzyl alcohol — in a sealed multi-dose vial with a stated lot and expiry. Brand matters less than what the label says.

Where can I buy bacteriostatic water for peptides?

From a laboratory or research supplier rather than a retail pharmacy. MyGLP1Store sells USP-grade BAC water in 10mL and 30mL vials, and includes one 10mL vial free with every peptide order.

For laboratory and research use only. Not for human or veterinary use. Nothing on this page is dosing guidance.