Bacteriostatic water and peptide reconstitution: a practical guide

What bacteriostatic water is

Bacteriostatic water is sterile water with 0.9% benzyl alcohol added as a preservative. The benzyl alcohol does not sterilise anything; it suppresses bacterial growth in solution that has already been opened, which is what allows one container to be entered several times over a period of days or weeks. That multi-withdrawal property is the whole reason it is the default diluent for a peptide stock that will be drawn from more than once.

Sterile water for injection is the same water without the preservative. Once its seal is broken there is nothing holding growth back, so it is treated as single-entry: open, use, discard the remainder. It is the right choice when a method requires a preservative-free vehicle, because benzyl alcohol is itself an assay-active compound at sufficient concentration and can affect cell viability at low dilution factors.

Saline is a different object again. Sodium-chloride diluents are not a validated solvent for some of the lipidated, acetate-buffered peptides we supply, including retatrutide, and have been associated with cloudiness and precipitation on addition. A vial that goes cloudy should be discarded, not filtered and used. Where a protocol page on this site names a diluent warning, that is what it is warning about.

The arithmetic: milligrams, millilitres and units

Reconstitution is one division. Milligrams of peptide in the vial divided by millilitres of solvent added gives the concentration in mg/mL. Every other number follows from it: the mass in any draw is the volume drawn multiplied by that concentration, and the volume for a given mass is that mass divided by the concentration.

U-100 syringes are graduated in units, where 100 units equals 1 mL. So 10 units is 0.1 mL and one unit is 0.01 mL. Units measure volume only; the mass a unit contains depends entirely on the concentration set at reconstitution.

One worked example, using a 10 mg vial and 2 mL of bacteriostatic water: 10 mg divided by 2 mL is 5 mg/mL. A 0.1 mL draw at that concentration contains 0.5 mg, which is 500 mcg, and 0.1 mL on a U-100 syringe reads as 10 units. Add 1 mL instead and the concentration doubles to 10 mg/mL, so the same 10 units now carries 1 mg.

To be explicit: this is arithmetic, not a dose. It describes what a given volume contains at a given concentration, and nothing about what quantity of any compound is appropriate for any purpose. Vial size matters to the sums too: a chart written for a 5 mg vial gives half the mass per unit of a 10 mg vial at the same volume, which is why every table on this site is computed from the vial size we actually ship rather than copied from a reference chart.

Choosing how much solvent to add

The solvent volume sets the concentration and, with it, how large the draws are. More water means a more dilute stock and a larger, easier-to-read volume for the same mass; less water means a concentrated stock and small draws where a one-unit error is a large percentage error. For small masses, a lower concentration is usually the more forgiving choice on a standard syringe.

The ceiling is the vial. Our vials accept up to about 3 mL comfortably; the volumes listed on each protocol page are the ones commonly used with that vial size. The floor is solubility and practicality, and at very high concentrations some peptides are slower to dissolve.

Two tools on this site do the sums for you. The peptide calculator at /tools/peptide-calculator/ takes a vial mass and a solvent volume and returns mg/mL, mass per 0.1 mL and mass per unit. Each product's protocol page carries the same table pre-computed for the vial size we ship, at the volumes typically used with it. Both are arithmetic on the numbers you enter; neither recommends a quantity.

Whatever volume you choose, write it on the vial label with the date. A stock whose concentration is not recorded is a stock whose draws cannot be interpreted.

Reconstitution technique, step by step

Bring the lyophilised vial and the bacteriostatic water to room temperature before opening either. Cold glass condenses moisture and a cold cake dissolves more slowly.

Remove the flip cap from each vial and swab both rubber closures with 70% isopropyl alcohol. Let them dry fully; alcohol carried in on a wet stopper is a contaminant.

Draw the chosen volume of bacteriostatic water into a sterile syringe. Insert the needle through the peptide vial's stopper at an angle so the tip rests against the inside of the glass wall, and let the water run slowly down the wall. A stream dropped directly onto the cake can foam it and shear the peptide.

Withdraw the needle and let the vial stand. Swirl gently or roll it between your palms if the cake is slow to clear. Do not shake: agitation creates foam and, for some peptides, aggregation that will not reverse.

Inspect against a bright background. A correctly reconstituted vial is clear and colourless, or very faintly coloured for a few compounds, with no particles and no haze. Cloudiness, particulates or a persistent film mean the vial is discarded rather than filtered, and the solvent choice revisited before the next one.

Label the vial with the concentration set and the date, then refrigerate. None of this involves taking a quantity of peptide out of the vial; that is a separate question this guide does not address.

Storage before and after reconstitution

As supplied, the lyophilised cake is the stable form. Sealed and dry it tolerates ambient temperature for the few days a domestic parcel spends in transit, which is why we ship without cold packs. On arrival it belongs at the temperature printed on the lot certificate, protected from light and moisture, and stays sealed until the day it is used.

Once reconstituted the picture changes. Peptide in solution degrades faster than dry cake, so the reconstituted vial belongs at 2 to 8 C in a refrigerator, not a freezer, because repeated freeze-thaw cycles damage most peptides in solution. The per-compound storage line on each protocol page, transcribed from the lot documentation, is the reference for how long a given reconstituted stock is expected to hold.

The bacteriostatic water itself has a separate clock. Once the container has been punctured, 28 days is the figure commonly cited in general laboratory practice for how long an opened container is considered usable. That is a convention, not a guarantee, and the manufacturer's labelling on the container is the document that governs. Whatever the date, discard water that has gone cloudy, coloured or particulate, and write the first-entry date on the container so nobody has to guess.

Buying bacteriostatic water in Canada

We stock bacteriostatic water in 10 mL sealed vials, listed at /products/bac-water/, and it ships from within Canada alongside any peptide order. Ten millilitres reconstitutes several vials at the volumes typically used, so one container is usually enough for a small batch of stocks.

Whoever you buy from, four things are worth checking. The container should be labelled sterile. The closure should be intact with the flip cap in place; a bare stopper has been entered or mishandled. The label should state 0.9% benzyl alcohol, because water sold without a preservative is sterile water, not bacteriostatic water, and behaves differently once opened. And lot documentation should be available. We send the manufacturer's certificate for a lot on request from customer service, and we describe it as exactly that: the manufacturer's QC document, matched to the lot number on the container.

The Canadian advantage is logistical rather than chemical. Bacteriostatic water is the same product on either side of the border; a domestic order simply arrives by Canada Post without a customs step, which matters when the parcel also contains lyophilised peptide you would rather not have sitting in a border hold.

Research use only, and what that means here

Everything on this page describes laboratory handling of research reference materials. The peptides we supply are for laboratory research use only. They are not approved by Health Canada for human use, carry no DIN or NPN, and are not supplied for any purpose other than research. Bacteriostatic water is sold as a laboratory diluent for the same reason.

That framing is why this guide stops where it does. It explains what concentration a given vial and volume produce, because that is arithmetic and the tables on this site would be unusable without it. It does not say what quantity of any compound is appropriate for anything, because that is not a question a research supplier is in a position to answer. Anyone with a question about a compound's effect on a person should ask a physician, not a website.

Every order is placed under a research-use declaration, and we hold to the other side of that bargain: no health claims, no outcome language, certificates described accurately, and a plain answer when a customer asks what a document does and does not show.

Frequently asked

What is the difference between bacteriostatic water and sterile water?

Bacteriostatic water is sterile water with 0.9% benzyl alcohol added as a preservative, which suppresses bacterial growth after the container has been opened and allows multiple withdrawals from one vial. Sterile water for injection has no preservative and is treated as single-entry: open, use, discard the remainder. Bacteriostatic water is the usual choice for a peptide stock drawn from more than once; sterile water is the choice where a method requires a preservative-free vehicle.

How much bacteriostatic water do I add to a peptide vial?

The volume sets the concentration, not a dose. Milligrams in the vial divided by millilitres added gives mg/mL: a 10 mg vial with 2 mL is 5 mg/mL, so 0.1 mL, or 10 units on a U-100 syringe, contains 500 mcg. More water makes a more dilute stock with larger, easier draws; less water the reverse. The calculator at /tools/peptide-calculator/ and the table on each protocol page do the arithmetic for the vial sizes we ship. Neither this guide nor those tools recommend a quantity of any compound.

How long does bacteriostatic water last once opened?

Twenty-eight days after first puncture is the period commonly cited in general laboratory practice, but that is a convention rather than a guarantee, and the manufacturer's labelling on the container governs. Store it as labelled, write the first-entry date on the vial, and discard it earlier if it turns cloudy, coloured or shows particles.

Can I use saline to reconstitute peptides?

Not as a default. Sodium-chloride diluents are not a validated solvent for some of the lipidated, acetate-buffered peptides we supply, retatrutide among them, and have been associated with cloudiness and precipitation on addition. Bacteriostatic water, or sterile water where a preservative-free vehicle is required, is the validated choice on every protocol page on this site. A vial that clouds on reconstitution should be discarded, not filtered.

Where can I buy bacteriostatic water in Canada?

We stock 10 mL sealed vials at /products/bac-water/, shipped from within Canada by Canada Post with no customs step. Whoever you buy from, check that the container is labelled sterile, the flip cap and closure are intact, the label states 0.9% benzyl alcohol, and lot documentation is available on request. Ours is the manufacturer's QC certificate for the lot, sent by customer service.

Referenced in our catalog