Bacteriostatic or Sterile Water: Choosing a Solvent
To reconstitute a lyophilized peptide, you dissolve the powder in a liquid. Two liquids come up more often than any other: bacteriostatic water and sterile water. Which one is used changes how long the finished solution stays usable.
Below we describe each, explain why bacteriostatic water is the common default, and note the cases where sterile water or a different solvent is mentioned instead.
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The two liquids
Sterile water for injection is purified, sterilised water with nothing added. It is meant for single use. After the seal is broken and liquid has been drawn out, nothing in it stops microbes from growing.
Bacteriostatic water is sterile water containing a small amount of benzyl alcohol, commonly 0.9%. Benzyl alcohol is bacteriostatic, meaning it holds back bacterial growth. That allows a multi-use vial to be accessed several times over a period without the contents spoiling as fast.
Why bacteriostatic water is the default
A reconstituted peptide vial is seldom used up in one go. It is usually accessed several times across days or weeks. The benzyl alcohol keeps microbes in check between uses. As a result, a refrigerated peptide solution prepared in bacteriostatic water usually remains usable well beyond one prepared in plain sterile water. That is why it is the solvent most often referenced for multi-use peptide vials.
Where sterile water or other solvents fit
Sterile water tends to be referenced for single-use work, or where benzyl alcohol is unwanted. A handful of peptides dissolve poorly and are referenced with other solvents, for instance a trace of acetic acid for stubborn sequences. For most research peptides, though, bacteriostatic water remains the standard reference solvent.
Handling notes
Whichever solvent you use, the same handling rules apply. Run the liquid slowly down the inner wall of the vial instead of firing it straight at the powder. Swirl gently and never shake. Keep the finished solution refrigerated and away from light. The full procedure is in our guide to reconstituting research peptides, and the reconstitution calculator works out concentration and draw volume for you.
Common questions
How long does a reconstituted peptide stay usable?
It depends on the peptide and the diluent. For an opened multi-dose vial made up with bacteriostatic water, a commonly used conservative rule is 28 days refrigerated at 2–8 °C. A solution in plain sterile water has a much shorter window, since it contains no preservative.
Can tap water or distilled water be used instead?
No. Reconstitution references call for pharmaceutical-grade sterile or bacteriostatic water only. Water from the tap or a distiller is not sterile, so it would contaminate the solution.