Both are clear, both come in glass vials, both are labeled "water for injection." The difference between bacteriostatic water and sterile water for injection (SWFI) is one preservative — 0.9% benzyl alcohol — and a 28-day shelf life. For peptide reconstitution, that gap matters.
What each one is
Sterile water (SWFI) is exactly what it sounds like: purified water that has been sterilized and packaged with no additives. Once opened and used, it has no protection against microbial growth — anything you reconstitute with it is considered single-use, immediate-discard.
Bacteriostatic water (BAC water) is sterile water plus 0.9% benzyl alcohol. Benzyl alcohol is mildly antimicrobial: it doesn't kill organisms aggressively, but it stops them from multiplying. That's what "bacteriostatic" means — stops bacteria, doesn't necessarily kill bacteria. Combined with refrigeration, this preservative gives a reconstituted multi-dose vial ~28 days of usable shelf life.
Why this matters for peptides
A typical peptide reconstitution is a multi-dose situation: you mix 2 mL into a 5 mg vial, then draw from it once a week for 4–5 months. Every time you puncture the rubber stopper with a needle, you're introducing a microscopic risk of contamination. Without a preservative, that risk compounds over days; with one, it doesn't.
That's the entire reason BAC water exists. Sterile water is for single-use applications — IV bags, one-time injections — where the vial is opened and used immediately, then discarded.
When to use each
Use BAC water for any peptide vial you'll draw from more than once.
That's the rule. The exceptions are rare and mostly clinical (some hospital IV preparations call specifically for SWFI to avoid the benzyl alcohol). For at-home or research reconstitution, BAC water is the default.
Practical tips
- Buy single 30 mL bottles, not large bulk containers. A 30 mL bottle is enough to reconstitute many small vials and lasts longer than any single peptide.
- Refrigerate after opening. Even BAC water keeps better cold. Most labels recommend discarding 28 days after first use.
- Don't reuse needles. The preservative slows growth — it doesn't eliminate the risk of cross-contamination from a dirty needle.
- Check the label for benzyl alcohol. Some "bacteriostatic" labels use methylparaben or other preservatives. For most peptides this is fine, but benzyl alcohol is the most studied and most widely recommended.
The 28-day rule, briefly
The 28-day shelf life is a guideline — not a guarantee. It assumes refrigerated storage at 2–8°C, clean handling, and a peptide that doesn't degrade independently. Some peptides (semaglutide, tirzepatide) are stable for the full 28 days; others (GHRP-6, some growth-hormone secretagogues) degrade faster regardless of preservative. The calculator on this site assumes 28 days as the default vial life, which is conservative for stable peptides and roughly right for most users.