Bacteriostatic water for peptides

Bacteriostatic (BAC) water is the default diluent for most research peptides. It is sterile water with 0.9% or 1.1% benzyl alcohol, a preservative that holds back bacterial growth. That is why you can draw from one mixed vial again and again. How long the vial lasts depends on the peptide. As for how much water to add, 1, 2 or 3 mL all work in a standard peptide vial. The water changes the concentration and the units per dose. The amount of peptide in the vial stays the same. The BAC Water page has the full profile and a calculator.

What the benzyl alcohol does

On the Hospira label, the 30 mL plastic vial carries 9 mg of benzyl alcohol per mL (0.9%). The 20 mL glass vial carries 11 mg/mL (1.1%). "Bacteriostatic" means the preservative stops bacteria from multiplying if some get in during use. It does not sterilize the vial. Keep the water itself at room temperature, 20 to 25 °C. Its label has no fridge instruction.

Benzyl alcohol also has some numbing effect. Five studies of facial botulinum toxin injections report less pain when the toxin is mixed in benzyl-alcohol saline instead of plain saline. In one split-face trial, all 15 patients felt less pain on the preserved side.

BAC water or sterile water

Sterile water for injection has no preservative. Its label marks the vial single-dose and says to discard the unused portion. BAC water is a multiple-dose product, made for repeated draws from one vial.

Tesamorelin shows the difference. It comes in a single-use form and a multi-day form. Egrifta SV is mixed with sterile water and used at once, and any leftover is thrown away. Egrifta WR is a reformulated powder mixed with BAC water, and one vial gives daily doses for 7 days. The active peptide is the same in both.

Sterile water still fits a few cases.

A vial used in one sitting. Nothing is left over, so there is nothing to preserve.
A known benzyl-alcohol allergy. Reactions are uncommon but reported, including one case confirmed by skin test.
A product that ships with its own diluent. The Egrifta SV label, for one, says to use only the sterile water provided.

Never use tap, distilled or spring water. Injection waters are labeled sterile and nonpyrogenic, meaning free of the contaminants that cause fever.

Benzyl alcohol can harm some folded proteins. Studies found it partly unfolds a model protein and makes interferon alpha-2a clump, more so at higher strength. A study of a 31-amino-acid acylated peptide found no clumping and no detectable interaction. Increlex, the drug form of IGF-1, is itself preserved with 9 mg/mL benzyl alcohol and keeps for 30 days after first use. BAC water stays the default diluent for most research peptides.

How much bacteriostatic water to add

Any volume works as long as the powder fully dissolves. More water lowers the concentration, so each dose takes more units on a U-100 syringe. The total peptide and the number of doses stay the same. The Novarel hCG label shows it with one vial. Mixed with 1 mL it is 10,000 IU/mL. Mixed with 10 mL it is 1,000 IU/mL.

Water added	Strength of a 10 mg vial	Units for a 250 mcg dose
1 mL	10 mg/mL	2.5 units
2 mL	5 mg/mL	5 units
3 mL	3.33 mg/mL	7.5 units

Every row holds 40 doses of 250 mcg. Pick the volume that turns your dose into an easy number of units to read. A standard peptide vial holds 3 mL at most. Glutathione and NAD+ vials hold up to 10 mL and take more water.

The calculator on the BAC Water page turns any vial size, water volume and dose into syringe units. The reconstitution and syringe-unit math guide covers the formula behind it.

Why a vial foams, clouds, gels or won't dissolve

What you see in the vial usually points to the cause. The growth-hormone reconstitution guide covers each one in depth.

What you see	Usual cause	Read more
Foam or froth after mixing	Shaking. Air-water surfaces make folded proteins unfold and clump. The Egrifta WR label expects some foam even after swirling, and the foam separates in a few seconds.	Growth-hormone reconstitution guide
Haze that clears in a minute or two	Powder dispersing before it dissolves. This is normal.	Reconstitution and syringe-unit math guide
A thick or gel-like vial	A long, water-repellent peptide such as tesamorelin, often in too little water	Growth-hormone reconstitution guide
Clumps that never dissolve	IGF-1 and analogs such as IGF-1 LR3, which need an acidic diluent	Growth-hormone reconstitution guide
Cloudy, gritty or colored after it was clear	Breakdown or contamination. Discard the vial.	Peptide storage and stability guide

The Genotropin, Pregnyl and tesamorelin labels all say not to shake the vial. The tesamorelin labels say to swirl or roll it instead. The Genotropin label says why. Shaking "may cause denaturation of the active ingredient."

Why glutathione won't dissolve

Glutathione dissolves easily, and its listed solubility in water is 292.5 mg/mL. The problem is too much powder in too little water. A 600 mg vial in 6 mL is 100 mg/mL and clears with swirling and a little time. The same vial in 2 mL is 300 mg/mL, past that limit. More water fixes it, and a glutathione vial takes up to 10 mL. The syringe-unit math guide explains the milky "snow globe" look in the first minute.

How long a mixed vial lasts

Two separate limits apply. An opened BAC water vial gets 28 days after its first puncture, unless its maker says otherwise. That is the standard rule for any opened multiple-dose container. The peptide you mixed has its own limit. Drug labels show how far apart the two can be.

Product	Diluent	Life once mixed or opened, per the label
Egrifta SV (tesamorelin)	Sterile water	Used at once, the rest discarded
Egrifta WR (tesamorelin)	BAC water	7 days at room temperature
Genotropin 5 mg and 12 mg cartridges (growth hormone)	Preserved with metacresol	28 days in the fridge
Novarel (hCG)	BAC water	30 days in the fridge
Increlex (IGF-1)	Ready-made solution with benzyl alcohol	30 days in the fridge after first use
Pregnyl (hCG)	Solvent with benzyl alcohol and salt	60 days in the fridge

People keep a mixed research peptide at 2 to 8 °C, away from light, and never freeze it. The Genotropin, Increlex and Pregnyl labels all say not to freeze. In a 2026 study, benzyl alcohol made freeze-thaw clumping of an antibody worse. For a mixed research peptide, the common window is up to 28 days in the fridge, the same as the opened-water limit. The peptide storage and stability guide has the details.

Questions people ask

How much bacteriostatic water should I add to a peptide? Any amount up to the 3 mL a standard vial holds, as long as the powder dissolves. Glutathione needs more, up to 10 mL. More water means more units per dose, with the same total peptide. The calculator on the BAC Water page turns your vial, water and dose into syringe units.

Can I use sterile water instead? Yes, for a vial you use in one sitting or if you react to benzyl alcohol. It has no preservative, so you throw away whatever is left.

Does bacteriostatic water need to be refrigerated? The water itself does not. Its label stores it at 20 to 25 °C and has no fridge instruction. A mixed research peptide vial is the one that goes in the fridge, at 2 to 8 °C.

How long is bacteriostatic water good after opening? It is good for 28 days after the first puncture, unless its label gives a different date.

How long does a mixed peptide vial last? Up to 28 days at 2 to 8 °C is the common window. The peptide can set a shorter one. Drug labels range from single use for Egrifta SV to 60 days for Pregnyl.

Does more water make the peptide weaker? No. The vial holds the same amount of peptide. Each dose is a larger volume, so it takes more units on the syringe.

Does benzyl alcohol damage peptides? It can make some folded proteins clump. A study of a short acylated peptide found no effect. Increlex, the drug form of IGF-1, contains it and keeps 30 days after opening.