Bacteriostatic water: what it is and how much to use
Bacteriostatic water is sterile water with 0.9% benzyl alcohol for multi-dose vials. What the label says, how volume sets concentration, and the math.
Key takeaways
- Bacteriostatic water (USP) is sterile water for injection with 0.9% benzyl alcohol as a preservative.
- The preservative allows repeated withdrawals from a multi-dose vial; sterile water without it is meant for single use.
- The FDA-listed label carries a boxed-style warning: not for use in neonates, because of benzyl alcohol toxicity.
- The volume you add does not change the amount of peptide, only the concentration (mg per mL).
Most research peptides ship as a freeze-dried (lyophilized) powder. Before use in a lab they are reconstituted, usually with bacteriostatic water.
What the label says
The FDA-listed Hospira label describes bacteriostatic water as “water for injection containing 0.9% (9 mg/mL) or 1.1% (11 mg/mL) of benzyl alcohol added as a bacteriostatic preservative”, supplied in a multiple-dose container “from which repeated withdrawals may be made”. Its pH is 5.7 (range 4.5 to 7.0).
The same label is headed “WARNING: NOT FOR USE IN NEONATES”: benzyl alcohol can be toxic to newborns.
Volume sets concentration, not amount
A 5 mg vial holds 5 mg whatever you add to it. The volume only decides concentration:
| Water added | Concentration (5 mg vial) | Concentration (10 mg vial) |
|---|---|---|
| 1 mL | 5 mg/mL | 10 mg/mL |
| 2 mL | 2.5 mg/mL | 5 mg/mL |
| 3 mL | 1.67 mg/mL | 3.33 mg/mL |
On a U-100 insulin syringe, 100 units equal 1 mL, so at 2.5 mg/mL each 10 units hold 0.25 mg. Run any vial size through the bacteriostatic water calculator or the peptide calculator, or see ready-made tables such as TB-500 10 mg on the protocol pages.
Related
What the evidence does not show
- Dosing. We do not give human dosing advice; doses on this site come only from FDA labels and trial protocols.
Frequently asked questions
How much bacteriostatic water do I add to a 5 mg vial?
Any volume works mathematically: 1 mL gives 5 mg/mL, 2 mL gives 2.5 mg/mL. The choice only changes how much liquid each dose occupies. Use the bacteriostatic water calculator to see the table.
Is bacteriostatic water the same as sterile water?
No. Sterile water for injection has no preservative and is intended for single use; bacteriostatic water contains benzyl alcohol so a vial can be entered more than once.
Can bacteriostatic water be used for babies?
No. The product label states it is not for use in neonates because benzyl alcohol can be toxic to them.
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