BAC water is the diluent. The volume you choose decides the concentration of every dose you’ll ever draw. Pick well once and the calculator does the rest — pick badly and you’re either drawing slivers of liquid or running out before week three.
What bacteriostatic water actually is
Bacteriostatic water for injection is purified water containing 0.9% benzyl alcohol as a bacteriostatic preservative. The preservative doesn’t sterilise the water — it stops bacteria from multiplying in it. That’s the bit that lets a reconstituted peptide vial sit in the fridge for weeks instead of needing to be used within a day.
It’s a generic pharmaceutical product. You’ll see it sold as multi-dose vials of 10 mL or 30 mL, sometimes called “BAC water”, “bacteriostatic water for injection” or “0.9% benzyl alcohol water”. They’re all the same thing.
Bacteriostatic vs sterile vs distilled — which one to use
Bacteriostatic water
0.9% benzyl alcohol preservative. Multi-dose use. Reconstituted vial stable up to 28 days refrigerated. This is what you want.
Sterile water
No preservative. Single-dose. Reconstituted vial good for 24–48 hours before bacterial load makes it unsafe.
Distilled water
Not pharmaceutical-grade. Not sterile. Not suitable for reconstituting research peptides.
Tap or filtered water
Contains minerals, chlorine, and bacteria. Never use for reconstitution.
How much to add — the trade-offs
There’s no single “correct” BAC water volume. You’re balancing three things:
- Dose precision. If your draw is 2 units on a U-100 syringe, a small reading error is a big percentage error. Aim higher.
- Concentration. More water = more dilute = larger draws. Less water = more concentrated = smaller draws.
- Vial longevity. The longer it takes to use the vial, the more important the 28-day shelf-life window becomes.
Worked examples
Worked for a target dose of 250 µg on a standard U-100 insulin syringe (1 unit = 0.01 mL):
| Vial size | BAC water | Concentration | Draw for 250 µg | Verdict |
|---|---|---|---|---|
| 5 mg | 1 mL | 5 mg/mL | 5 units | Too small — measurement error compounds |
| 5 mg | 2 mL | 2.5 mg/mL | 10 units | ✓ Sweet spot |
| 5 mg | 3 mL | 1.67 mg/mL | 15 units | ✓ Comfortable |
| 5 mg | 5 mL | 1 mg/mL | 25 units | ✓ Easy to read, larger draw |
| 10 mg | 2 mL | 5 mg/mL | 5 units | Too small for 250 µg target |
| 10 mg | 4 mL | 2.5 mg/mL | 10 units | ✓ Sweet spot |
Buying & storing BAC water
How much do you need per vial?
A 30 mL multi-dose vial of BAC water reconstitutes roughly 6–10 average research vials, depending on volume choice. One bottle of BAC water will usually outlast 3–5 weeks of moderate research use.
Storage
Unopened: room temperature, away from sunlight. Opened: refrigerate, and use within the manufacturer’s stated post-puncture window (usually 28 days). Don’t share a BAC water vial across multiple peptide reconstitutions if any time has passed between them — repeated stopper punctures increase contamination risk.