Bacteriostatic water vs saline for peptides

Bacteriostatic water is the standard diluent for research peptides, and saline is not. Bacteriostatic water is sterile water with 0.9% benzyl alcohol added as a preservative, so a reconstituted vial can be drawn from over several weeks in the fridge. Saline is sterile water with 0.9% sodium chloride and, in its plain form, no preservative at all. For peptide work that makes it closer to sterile water than to bacteriostatic water: fine for a vial used in one sitting, a poor fit for one you plan to keep.

The confusion usually comes from a third product with a similar name, bacteriostatic sodium chloride. It has the preservative and the salt, and it is not the same thing as bacteriostatic water. The sections below cover all three.

Bacteriostatic water, saline and bacteriostatic sodium chloride compared

The difference between these liquids comes down to two ingredients: a preservative and salt.

Sterile water, the fourth option, has neither the preservative nor the salt. Bacteriostatic water vs sterile water covers that comparison.

Is bacteriostatic water the same as bacteriostatic sodium chloride?

No. Bacteriostatic sodium chloride is saline with a preservative added. Bacteriostatic water is water with a preservative added and no salt. The two share the word "bacteriostatic" because both contain benzyl alcohol, which slows microbial growth after the stopper has been punctured.

In practice:

• Read the label, not the first word. "Bacteriostatic" tells you there is a preservative. It says nothing about salt.
• Research peptide handling guides, including ours, assume bacteriostatic water unless they say otherwise.
• If a supplier ships "bac water" that turns out to be bacteriostatic sodium chloride, you have a salt solution, not the diluent the guide assumed.

Why saline is rarely used to reconstitute research peptides

Most peptides dissolve in saline without trouble. It has two drawbacks for vial storage, though.

The first is that plain saline has no preservative. It is sterile until the vial is opened, and after that it is no better protected than plain water, so a peptide dissolved in it should be treated as single-use. The benzyl alcohol in bacteriostatic water is what lets a vial go back into the fridge and be drawn from again.

The second is salt. Some sequences, especially more hydrophobic ones, can be harder to dissolve in a salt solution than in plain water. If a solution turns cloudy or powder stays undissolved, you want to be able to rule out the diluent and look at the material, which is simpler when the diluent is the one the handling guides assume.

Bacteriostatic water has neither drawback, which is why the reconstitution guides on this site use it throughout.

Concentration is the same whichever diluent you pick

The diluent does not change the arithmetic. Concentration is the amount of peptide in the vial divided by the volume of liquid added:

Salt adds to the solution, but the peptide amount stays the same. The technique does not change either: run the liquid down the glass wall and swirl rather than shake. How to reconstitute a lyophilized peptide covers each step, and how to reconstitute glutathione shows how the volumes scale for a large vial.

Choosing bacteriostatic water in Canada

Bacteriostatic water is the only diluent pepuro sells. Bacteriostatic water comes in 3, 10, 20 and 30 mL vials. The 30 mL vial is Pfizer (Hospira) brand, and the smaller sizes are generic. Each one is sterile water with 0.9% benzyl alcohol and no sodium chloride. It is a diluent, not a peptide, so it is not purity tested by HPLC the way a peptide vial is.

Kept cold, a vial reconstituted with it holds for weeks, not months. How to store research peptides gives the temperatures. Your concentration is only as accurate as the milligram amount on the peptide vial, and how to read a peptide lab report shows how to confirm that amount.

Every order from pepuro ships free from our Calgary, Toronto and Vancouver distribution centers, the same business day your e-Transfer clears, and every peptide vial carries a QR code that opens its own third-party lab report.

For research use only. Not for human or animal consumption.