Bacteriostatic water
What it is, how it differs from sterile water, how long an opened vial lasts, and when a label calls for something else.
What it is
Bacteriostatic water for injection is sterile water with 0.9% benzyl alcohol (9 mg per mL) added as a preservative. The preservative keeps bacteria from multiplying, so the vial can be punctured again and again. Its label allows one use: diluting or dissolving a drug for injection under the skin, into a muscle or into a vein, following that drug’s own instructions. It isn’t meant to be injected by itself.
It’s the usual liquid for mixing multi-dose peptide vials, because the preservative goes on protecting the mixed vial between doses. It’s also sterile and nonpyrogenic, free of the bacterial toxins that cause fever; tap, bottled and distilled water aren’t made to either standard.
Bacteriostatic or sterile water
| Bacteriostatic water | Sterile water | |
|---|---|---|
| Preservative | 0.9% benzyl alcohol | None |
| Vial | Multiple-dose | Single-dose only |
| After the first puncture | Throw it out 28 days later, unless the maker says otherwise (USP 797) | Use it once and throw out the rest |
| A peptide vial mixed with it | Several doses, kept in the fridge | One dose |
| Newborns | Never: benzyl alcohol is toxic to them | The labels’ choice when water is needed for newborns |
| Before opening | 20 to 25 °C (68 to 77 °F) | 20 to 25 °C (68 to 77 °F) |
Bacteriostatic water’s pH is 5.7 (range 4.5 to 7.0). Some products use 1.1% benzyl alcohol (11 mg per mL).
When a label calls for something else
The drug’s own instructions decide the liquid. On this site, 6 compounds have a label or maker that names something other than bacteriostatic water:
- TesamorelinApproved drug
- Egrifta SV is mixed with sterile water and used right away. Egrifta WR is mixed with bacteriostatic water and kept at room temperature for up to 7 days.
- IGF-1 LR3Preclinical
- Reagent makers recommend dissolving it in about 0.6% (100 mM) acetic acid so it doesn’t stick to surfaces. Community guides split between acetic acid and bacteriostatic water. Acidic solutions sting.
- GonadorelinApproved drug
- Bacteriostatic water. A 2 mg vial in 2 mL gives 1 mg/mL, so 100 mcg is 10 units on a U-100 syringe. The approved 100 mcg diagnostic vial is different: it is mixed with 1 mL of sterile water, used at once, and any remainder discarded.
- TriptorelinApproved drug
- Research vials of the acetate are mixed with bacteriostatic water: 2 mg in 2 mL gives 1 mg/mL, so 100 mcg is 10 units on a U-100 syringe. The approved depots are a different product and cannot be measured this way: they are lyophilized microgranules, mixed with the 2 mL of sterile water supplied in the kit, shaken for 30 seconds and injected into muscle within 2 minutes, at strengths of 3.75, 11.25 and 22.5 mg.
- Thymosin Alpha-1Approved drug
- The label vial (1.6 mg) is mixed with 1 mL of sterile water and used once, within 24 hours (Italy’s label). Multi-dose research vials are usually mixed with bacteriostatic water.
- ThymalinApproved drug
- The label dissolves each vial in 1–2 mL of 0.9% sodium chloride just before the IM injection. One vial is one dose.
Ready-made pens and solutions, such as semaglutide’s and tirzepatide’s, need no mixing.
Using a vial safely
- Use it only if it’s clear and the seal is intact.
- Wipe the rubber stopper with an alcohol swab and let it dry before every puncture.
- Use a new, sterile needle and syringe every time you enter the vial, even for your own next dose.
- Write the date of the first puncture on the vial, and throw it out 28 days later.
- Keep the vial for one person; don’t share it.
How much to add
The amount of water sets the strength, not the dose: more water means more units for the same dose. Each compound’s page and its vial pages give common amounts for each vial size, and the calculator turns any amount into syringe units. The reconstitution guide walks through the mixing.
Common questions
For a single dose, yes: mix it, use it right away and throw out the rest, since there’s no preservative. Some labels require sterile water (above).
Unopened, it keeps until the expiration date on the vial, stored at 20 to 25 °C. Once punctured, 28 days.
Not before opening; its label says room temperature. A peptide vial mixed with it goes in the fridge.
Its label rules it out for newborns, in whom benzyl alcohol has caused toxicity, and for injections around the spine (epidural or spinal).
Sources: Hospira labels for Bacteriostatic Water for Injection (revised 2019) and Sterile Water for Injection (revised 2025); USP general chapter 797; CDC injection safety checklist; each compound’s label or maker’s instructions, cited on its page.