Bacteriostatic water is sterile water with a preservative, benzyl alcohol, that slows the growth of bacteria. That lets the vial be punctured more than once. Whether it is the right liquid for a given peptide depends on the peptide’s label, not the water’s. Its own label says so.
This page reads the labels for both kinds of water and for two peptide products. It gives no mixing volumes of its own. Research-only peptides such as BPC-157 have no label, so no approved diluent exists for them.
What the label says it is
Bacteriostatic Water for Injection, USP is sterile, nonpyrogenic water containing 0.9% (9 mg/mL) or 1.1% (11 mg/mL) benzyl alcohol as a preservative [1]. It comes in a multiple-dose container “from which repeated withdrawals may be made.” The labeled vials hold 30 mL or 20 mL and are stored at 20°C to 25°C.
Its only indication is diluting or dissolving drugs for intravenous, intramuscular or subcutaneous injection. The label adds a condition: this is done “according to instructions of the manufacturer of the drug to be administered.” It also says some drugs may be incompatible with a vehicle containing benzyl alcohol.
How it differs from sterile water
Sterile Water for Injection, USP contains no bacteriostat, antimicrobial agent or buffer [2]. Its label says it is intended only for single-dose injection after mixing. When less than the full amount is needed, the unused portion is discarded.
So the practical difference is the preservative. Bacteriostatic water can be drawn from repeatedly. Sterile water is used once.
When a peptide label names a different diluent
The label for EGRIFTA SV, a tesamorelin product, is explicit: use only the diluent provided, which is Sterile Water for Injection [3]. The mixed solution is injected immediately, and any leftover solution and diluent are thrown away. The label says not to refrigerate or freeze the mixed solution.
The label for GENOTROPIN, a somatropin product, uses a third approach [4]. Its cartridge holds the powder and a diluent preserved with metacresol, not benzyl alcohol. Once mixed, it is refrigerated at 2°C to 8°C for up to 28 days.
What the label does not say
The bacteriostatic water label does not state how long a vial may be used once opened. It does say not to store reconstituted drug solutions unless the drug’s manufacturer directs it [1]. It lists possible reactions from the solution, the added drug or the technique, including fever, local tenderness, abscess and infection at the site.
The steps that follow adding the water are covered in how to reconstitute peptides, and where the finished dose goes in peptide injection sites. For a prescribed product, the dispensing pharmacist can confirm which diluent its label requires. How this site sources a protocol page is set out in the methodology.