Bacteriostatic water

Bacteriostatic water is sterile water with a small amount of preservative added. The standard product, Bacteriostatic Water for Injection, contains 0.9% benzyl alcohol (9 mg per mL). “Bacteriostatic” means the preservative stops bacteria from multiplying; it does not kill everything, and it does not make a contaminated vial safe.

It is used to dissolve or dilute medicines and other powders, including freeze-dried peptides, in vials that will be used more than once. The preservative is the whole point: each time a needle goes through the stopper there is a chance of introducing bacteria, and benzyl alcohol keeps them from growing between uses.

The two look identical but behave differently:

• Bacteriostatic water contains benzyl alcohol, comes in multi-dose vials and is intended for repeated use over a limited period.
• Sterile water for injection contains no preservative. It is sterile until opened, and is intended for single use: once the seal is broken, there is nothing to stop bacterial growth.

That difference matters for storage. A peptide mixed with sterile water should be treated as single-use; one mixed with bacteriostatic water can be kept in the fridge and used over a longer period, subject to the stability of the peptide itself. Our guide to storing peptides covers both.

A few peptides dissolve poorly in water and are reconstituted with other solvents, such as dilute acetic acid, in laboratory settings. Where that applies, the individual peptide page or the supplier’s instructions should say so.

Saline is water with 0.9% sodium chloride, the same salt concentration as body fluids. It also comes in a bacteriostatic version, containing benzyl alcohol as a preservative. Saline is used as a diluent for some products, but salt can affect how some peptides dissolve and how stable they are in solution. Most research peptide suppliers and instructions refer to bacteriostatic water rather than saline; where a peptide needs something different, it should say so.

Unopened vials are normally stored at controlled room temperature, away from light, until the printed expiry date; check the label, as storage directions can differ between manufacturers. Once the stopper has been punctured, the vial has a limited in-use life (below). Keep the stopper clean, and do not use a vial that has been left open, has changed colour or contains particles.

For multi-dose vials of injectable products, a common pharmacy standard is to discard the vial 28 days after it is first punctured, unless the manufacturer states a different period. Unopened vials keep until the expiry date printed on them, stored as the label says (usually at controlled room temperature).

Keep in mind that there are two separate clocks once you have made up a peptide: how long the water stays usable, and how long the peptide stays intact in solution. The shorter of the two applies.

Adding water does not change how much peptide is in the vial. It only changes how concentrated the solution is. A worked example:

• A 5 mg vial with 2 mL of water gives 2.5 mg per mL, which is 2,500 mcg per mL.
• A U-100 insulin syringe is marked in units, with 100 units per mL. So 1 unit is 0.01 mL.
• At 2,500 mcg per mL, each unit holds 25 mcg, and 10 units (0.1 mL) holds 250 mcg.
• Add 1 mL instead of 2 and the solution is twice as strong: each unit then holds 50 mcg.

The peptide calculator does this arithmetic for any vial size and water volume. One caution it cannot fix: the calculation assumes the vial contains exactly what the label says. If a vial is under-filled, every figure is off by the same proportion. That is why quantity testing matters; see how to read a COA.

• Not for newborns. Benzyl alcohol has been linked to a serious toxic reaction in premature infants, so products containing it carry a warning against use in neonates.
• A diluent, not a product in itself. Bacteriostatic water is intended for dissolving or diluting other products, not for injection on its own: water with nothing dissolved in it is hypotonic.
• Check the seal and the expiry date, and do not use a vial that is cloudy or has particles in it.
• Buy from a source that states what it is. The label should state the benzyl alcohol content, the volume, a batch number and an expiry date.

Bacteriostatic water is a pharmaceutical product in its own right. How it is sold differs between countries: in some it is available from pharmacies, in others mainly through medical suppliers or online. When you buy it, check that the label names the manufacturer and states the benzyl alcohol content, volume, batch number and expiry date; that the flip-off cap and seal are intact; and that the liquid is clear. Unlabelled vials, or vials relabelled by a reseller with no manufacturer named, give you no way to know what you have.

Vials come in several sizes, commonly 10 mL or 30 mL. A larger vial is only better value if you will use it within its in-use period once opened; otherwise the unused water is wasted. Many peptide suppliers sell bacteriostatic water alongside their catalogue. The price is rarely what separates suppliers; delivery, testing and service usually matter more. Compare suppliers on those in the supplier directory or the best-rated ranking.