May 25, 2026
Bacteriostatic Water (BAC Water) — Complete FAQ
What BAC water is, why it's different from sterile water, how to store it, and answers to the questions people ask the most.
Bacteriostatic water — usually called BAC water — is one of those background supplies that everyone tracking peptides relies on but few people understand. This FAQ answers the questions that come up most often.
What is BAC water?
BAC water is sterile water for injection with 0.9% benzyl alcohol added as a preservative. The benzyl alcohol inhibits bacterial and fungal growth, which is what lets you draw from the same vial multiple times over a period of weeks without contaminating the solution.
How is it different from "sterile water for injection"?
Plain sterile water for injection (SWFI) has no preservative. Once you puncture the seal, it's typically used within a few hours. BAC water buys you ~28 days of multi-use shelf life once opened.
| BAC water | Sterile water (SWFI) | |
|---|---|---|
| Preservative | Benzyl alcohol 0.9% | None |
| Multi-use? | Yes (~28 days) | No (single-use) |
Storage
- Unopened: room temperature, out of direct sunlight.
- Opened: refrigerate; ~28 days from first puncture.
- Label the vial with the date of first puncture.
Is BAC water safe for everyone?
Two specific cautions: neonates and infants should not receive benzyl alcohol-containing preparations, and allergies to benzyl alcohol are rare but real.
How much should I use per vial?
Pick a BAC volume that puts your typical dose between 10 and 50 units on a U-100 syringe. See the reconstitution calculator guide.
Can I reuse one BAC vial across peptide vials?
Yes — that's its purpose. Aseptic technique (alcohol-swab the rubber stopper, fresh needle for each draw) is what matters.
Common mistakes
- Drawing from a punctured vial after 28 days.
- Mixing with sterile water without recalculating concentration.
- Storing in the freezer.
- Cross-contaminating the rubber stopper.
- Does BAC water expire on the shelf?
- Unopened vials typically expire 18–24 months from manufacture. Check the label.
- Is 0.9% benzyl alcohol the same as 0.9% saline?
- No. Different molecule, different purpose.
Informational only. Not medical advice.