Bacteriostatic water: what it is and how a peptide is reconstituted

What bacteriostatic water is

The medical version, Bacteriostatic Water for Injection USP, has been FDA-approved since 1982 and is sold on prescription. Its label describes it as sterile, nonpyrogenic water with 0.9% benzyl alcohol as a bacteriostatic preservative, and limits its use to diluting or dissolving other drugs. Its pH is 5.7 (range 4.5 to 7.0).

The benzyl alcohol is there for after the vial is opened: it stops bacteria growing if the container is contaminated during use. That is why the same vial can be entered several times. Some approved lyophilized medicines, such as Egrifta WR (tesamorelin), include bacteriostatic water as the diluent in the package.

Bacteriostatic water, sterile water and saline

• Bacteriostatic water: water + 0.9% benzyl alcohol. Multiple-use vial.
• Sterile water: water only, no preservative. Single use: what is left is discarded.
• Saline 0.9%: water with sodium chloride, isotonic, no preservative.

Which one fits depends on the compound. A 2005 study found that dissolving a lyophilized protein in 0.9% benzyl alcohol caused more aggregation than dissolving it in water, so not every peptide or protein behaves the same way. Always check what the COA or the compound’s data sheet says.

How a peptide is reconstituted in the lab

Reconstituting means dissolving the lyophilized powder in a diluent to get a solution of known concentration. A typical laboratory procedure, with aseptic technique:

1. Let the peptide vial and the diluent reach room temperature before opening, so no moisture condenses on the powder.
2. Wipe the stoppers with 70% alcohol and let them dry.
3. Decide the concentration your assay needs and work out the volume: concentration (mg/mL) = peptide mass (mg) ÷ diluent volume (mL). For example, 10 mg in 2 mL gives 5 mg/mL.
4. Add the diluent slowly, letting it run down the vial wall rather than straight onto the powder.
5. Mix by gently swirling the vial. Do not shake hard: foam and agitation can damage some peptides.
6. Label the vial with the compound, lot, concentration and date, and store it as its data sheet says.

This describes a laboratory procedure. It is not a guide to preparing injections or to use in people or animals.

How long bacteriostatic water lasts

Unopened, the label says to store it at controlled room temperature (20 to 25 °C) and not to use it unless it is clear and the seal is intact. Once opened (entered), FDA’s guidance for multiple-dose containers gives a limit of 28 days, unless the manufacturer states otherwise.

Label warnings

The medical label opens with the warning "not for use in neonates": in 1982 CDC reported deaths of premature babies linked to flush solutions containing benzyl alcohol. It also warns that water without a solute must not be given intravenously, because it can cause hemolysis.

Where to buy bacteriostatic water in Puerto Rico

In pharmacies, the USP version is dispensed on prescription. For the laboratory, our bacteriostatic water comes in 3, 10 and 30 mL vials and ships from Puerto Rico with your peptides. It does not count toward the volume discount on peptides.