Peptide reconstitution 101

What bacteriostatic water (BAC water) is

Bacteriostatic water is sterile water for injection containing 0.9% benzyl alcohol as a preservative. The benzyl alcohol inhibits microbial growth, which means the same vial can be entered multiple times (multi-draw) without immediate contamination. Plain sterile water for injection does not contain a preservative and should be discarded after a single draw.

For research reconstitution, BAC water is the standard diluent. It is available from pharmacy suppliers, compound pharmacies, and some research chemical vendors. Confirm the source conforms to USP standards for water for injection.

The two formulas you need

Every reconstitution calculation reduces to two sequential steps: first compute concentration, then compute the volume to draw.

Worked example

Syringe illustration — 0.5 mL, 10 units

The dashed line above marks the 10-unit position on a 0.5 mL U-100 syringe — the draw volume for the worked example above (0.1 mL, 0.25 mg at 2.5 mg/mL concentration).

Common pitfalls

• mcg vs mg confusion — 1 mg equals 1,000 mcg. Peptide vials are often labeled in mg; peptide doses in research literature are sometimes stated in mcg. Always convert to the same unit before computing.
• Reading a U-40 syringe as if it were U-100 — a U-40 syringe has 40 units per mL, not 100. Ten units on a U-40 syringe is 0.25 mL, not 0.1 mL. This is a 2.5× error. Verify the syringe label before drawing.
• Drawing past the max-tick line — the graduated scale ends at the barrel's capacity. Attempting to draw beyond it introduces air into the solution.
• Reusing BAC water past 28 days — most references recommend discarding reconstituted vials and opened BAC water vials after 28 days of refrigerated storage, as benzyl alcohol efficacy diminishes over time.

Storage and handling

Most lyophilized (freeze-dried) peptides are stored at 2–8 °C before reconstitution. Once reconstituted with BAC water, vials are typically used within 28 days when refrigerated. Freezing a reconstituted vial is generally discouraged because freeze-thaw cycles can degrade peptide structure; consult the specific product's technical data sheet for guidance.

Keep vials away from direct light. When reconstituting, inject the BAC water against the glass wall of the vial rather than directly onto the lyophilized cake to minimize mechanical stress on the peptide.