BAC Water vs Sterile Water

The most common reconstitution question, answered without dogma.

What each actually is

Bacteriostatic water (BAC): sterile water with 0.9% benzyl alcohol. The preservative inhibits bacterial growth after the vial is punctured, allowing multiple draws over time.
Sterile water: water with nothing added. Sterile at the moment of production and at first puncture — after that, anything that enters stays.

When BAC is the right call

Any multi-dose protocol — reconstituting with BAC and dosing over days/weeks is the standard practice for research peptides.
Peptides that are stable in solution for weeks (most common research peptides tolerate BAC fine).
You want the flexibility to draw from the same vial repeatedly without exponential contamination risk.

When sterile water makes sense

Single-use preparations, or protocols where the entire vial is used immediately.
Compounds flagged as sensitive to benzyl alcohol — a small number of peptides are described in literature as incompatible with preservatives; check the specific peptide's guidance.

The contamination math

Every needle puncture is a contamination event. BAC doesn't sterilize — it slows bacterial growth between punctures. Sterile water doesn't even slow it. If you puncture a sterile-water vial more than once, you are serial-diluting risk into your own protocol. Most "my peptide went bad" stories trace back to solvent or handling, not the peptide.

Practical rules

Reconstitution for multi-day research use → BAC, refrigerated, use within ~30 days.
One-shot preparation → sterile water is acceptable; discard leftovers.
Never share needles, never reuse needles, always wipe the stopper with alcohol before every puncture.
Cloudy BAC or floating particles = contaminated or expired. Throw it out.