Bacteriostatic Water: The Complete Guide
You have a vial of freeze-dried powder and you need to turn it into something you can inject. That requires bacteriostatic water: sterile water with 0.9% benzyl alcohol, which stops bacteria from growing so you can puncture the same vial dozens of times over 28 days without it going bad. Every reconstitution starts here.
- Sterile water + 0.9% benzyl alcohol. That is the entire ingredient list.
- Dissolves freeze-dried compounds for injection. You can puncture the same vial for 28 days because the preservative keeps bacteria from growing.
- Unopened? Lasts years. After first puncture? 28 days in the fridge, then throw it out.
- Not the same as sterile water. Sterile water has no preservative, one puncture only. Mix them up and you are injecting bacteria by day three.
How the preservative actually works
Benzyl alcohol disrupts bacterial cell membranes. At 0.9% it is bacteriostatic, which means it stops bacteria from reproducing. It does not kill them outright. That distinction matters: if you introduce bacteria through a dirty needle or by touching the stopper with your fingers, the benzyl alcohol slows the problem. It does not fix it.
The preservative is a safety net, not a substitute for clean prep. Most "bad reactions" to reconstituted compounds are not reactions to the compound. They are reactions to contamination that happened during prep. The compound was fine. The hands were not.
Bacteriostatic water vs sterile water
People confuse these constantly. A pharmacist in Bangkok once handed someone sterile water when they asked for bac water and said "same thing." It is not the same thing. One ingredient difference, but it changes everything about how you use the vial.
| Property | Bacteriostatic water | Sterile water |
|---|---|---|
| Preservative | 0.9% benzyl alcohol | None |
| Multi-use | Yes - puncture multiple times over 28 days | No - single puncture, discard remainder |
| Shelf life (open) | 28 days refrigerated | Use immediately, discard rest |
| Best for | Peptide reconstitution (multi-dose vials) | Single-use medications, neonatal use |
| Neonatal safe | No - benzyl alcohol toxic to newborns | Yes |
| Cost (30 mL) | $5-15 | $3-10 |
If you are reconstituting a peptide vial that you will draw from more than once, use bacteriostatic water. If you are using the entire vial in a single injection, sterile water works. In practice, almost nobody uses an entire vial in one shot, so bacteriostatic water is the default for peptide protocols.
How to use bacteriostatic water for reconstitution
Your first reconstitution feels like defusing a bomb. It is not. It takes under five minutes and the only way to genuinely mess it up is to shake the vial (don't) or spray water directly onto the powder (also don't). Everything else is just being clean and going slow.
Wipe the rubber stopper of both vials with a fresh alcohol swab. Let air dry for 10 seconds. Do not blow on them (your breath is not sterile, despite what your confidence suggests). Fresh swab for each vial.
Pull the plunger back to your target volume (usually 2 mL). Push the needle into the bac water vial, inject that air (equalizes pressure so you are not fighting a vacuum), flip it upside down, and draw to your mark. Tap out air bubbles. If you are unsure how much to add, the calculator has your compound preloaded.
Needle into the compound vial. Angle it so the tip touches the glass wall, not the powder cake. Push the plunger slowly. Let the water trickle down the inside wall. Do not blast it onto the powder like you are pressure-washing a driveway. The compound is fragile. Treat it like something you paid money for, because you did.
Roll the vial between your palms or tilt it slowly side to side. Never shake. Shaking can denature the compound, which is a fancy way of saying you just broke the thing you paid for. The powder should dissolve into a clear solution within 30-60 seconds. If it does not, put it in the fridge for 10 minutes and come back. Patience, not force.
Write today's date on the vial with a marker. This is your Day 1. Store in the refrigerator at 2-8°C (36-46°F). Use within 28 days of reconstitution. The reconstituted solution is now the clock you are racing.
How much bacteriostatic water to add: common compounds
Every Reddit thread about reconstitution has someone doing different math. The water volume does not change the total dose in the vial. It only changes how concentrated the solution is. More water = more dilute = easier to measure small doses on an insulin syringe. Less water = you are squinting at the difference between 4 and 5 units.
Here is what most people actually use. The calculator does the math for any combination.
| Compound | Vial size | BAC water | Concentration | Common dose | Syringe units |
|---|---|---|---|---|---|
| BPC-157 | 5 mg | 2 mL | 2.5 mg/mL | 250-500 mcg | 10-20 units |
| TB-500 | 5 mg | 2 mL | 2.5 mg/mL | 2.5 mg (loading) | 100 units (1 mL) |
| Semaglutide | 5 mg | 2 mL | 2.5 mg/mL | 0.25-1 mg | 10-40 units |
| Tirzepatide | 10 mg | 2 mL | 5 mg/mL | 2.5-5 mg | 50-100 units |
| Retatrutide | 10 mg | 2 mL | 5 mg/mL | 1-4 mg | 20-80 units |
| Ipamorelin | 5 mg | 2.5 mL | 2 mg/mL | 200-300 mcg | 10-15 units |
| CJC-1295 | 5 mg | 2.5 mL | 2 mg/mL | 100 mcg | 5 units |
| GHK-Cu | 5 mg | 2 mL | 2.5 mg/mL | 200-500 mcg | 8-20 units |
| PT-141 | 10 mg | 2 mL | 5 mg/mL | 1-2 mg | 20-40 units |
| Sermorelin | 5 mg | 2.5 mL | 2 mg/mL | 200-300 mcg | 10-15 units |
| MOTS-c | 10 mg | 2 mL | 5 mg/mL | 5-10 mg | 100-200 units |
| KLOW | 80 mg blend | 4 mL | 20 mg/mL | Per protocol | Per protocol |
5 mg vial + 2 mL bac water = 2.5 mg per mL. Your insulin syringe marks 0.1 mL as "10 units." So 250 mcg = 10 units on the syringe. Want 500 mcg? Draw to 20 units. That is the whole system. The calculator does it for any compound at any volume, so you do not have to do division at 10 PM with a syringe in your hand.
How to store bacteriostatic water
The rules change the moment you put a needle through the stopper for the first time. Before that, it is one of the most low-maintenance things you own. After that, the clock starts.
- Cloudy. Fresh bac water is perfectly clear. Any haziness = contamination. Done.
- Floaters or film. Specks, particles, anything on the surface. Discard.
- Any color at all. It should be colorless. Yellow, brown, pink tint = throw it out.
- Rubber in the liquid. This is called stopper coring. Happens when you use dull needles or puncture the same spot repeatedly. If you see tiny rubber fragments floating in there, the vial is done.
- Smells wrong. Bac water has a faint alcohol smell. That is normal. Anything else is not.
When in doubt, throw it out. A 30 mL vial costs under $10. An infection costs infinitely more.
How long does bacteriostatic water last? The 28-day rule
Everyone asks if 28 days is real or conservative. It is a USP standard, not someone's guess. Every needle puncture introduces a tiny amount of contamination. The benzyl alcohol suppresses it. But it cannot suppress it forever. After 28 days of repeated punctures, the cumulative bacterial load crosses the line where the preservative stops winning.
Until expiry date (2-3 years from manufacture). Store at room temperature. No degradation concern.
First puncture. The 28-day clock starts now. Refrigerate. Write the date on the vial.
Safe to use. Benzyl alcohol is actively suppressing bacterial growth. Use a fresh needle each draw. Refrigerate between uses.
Discard. Even if there is water remaining. Even if it looks clear. The cumulative contamination risk from repeated punctures exceeds the preservative's capacity. A new vial costs $5-10. An abscess costs a hospital visit.
Does the benzyl alcohol itself expire? No. It is chemically stable for years. The 28-day limit is not about the preservative degrading. It is about the accumulated bacteria from every needle puncture eventually outnumbering what 0.9% benzyl alcohol can hold back. The preservative is still working on day 29. It is just losing the fight.
Where to buy bacteriostatic water
The short answer is online. The long answer involves a pharmacy counter, a confused pharmacist, and you explaining what benzyl alcohol is while the line behind you grows. Here is what actually works.
Do you need a prescription? Technically, yes. The FDA classifies it as Rx. In practice, you can order it from a dozen medical supply websites right now without one. Your pharmacist might ask, might not. The enforcement is inconsistent in a way that frustrates everyone on both sides. Outside the US, rules vary by country.
- Hospira (now Pfizer) is the gold standard manufacturer. Fresenius Kabi is also legitimate. If the vial has no manufacturer name, that is your first red flag.
- The label should say 0.9% w/v benzyl alcohol. Different percentage? Different product. Walk away.
- Glass vial, flip-off cap over the rubber stopper, factory sealed. If the cap looks hand-crimped or bent, someone has been inside that vial before you.
- Lot number and expiry date on the label. Both. If either is missing, you cannot verify what you are holding.
- A COA (Certificate of Analysis) confirms sterility and benzyl alcohol concentration. Not every retailer provides one, but the ones worth buying from do. How to read a COA.
In Thailand: Bac water is available from pharmacies and online sellers. See our Bangkok-specific guide for pharmacies, prices, and same-day delivery options.
Who should not use this
- Reusing needles. Every draw, fresh needle. Reused needles are dull (hurts more), contaminated (defeats the entire point of the preservative), and chew up the stopper faster.
- Skipping the alcohol swab. Two seconds. That is what stands between you and an injection site infection. Non-negotiable.
- Blasting water onto the powder. Trickle it down the glass wall. The compound is not a dish you are rinsing.
- Shaking. Swirl. That is it. If you shake it, you are paying to denature what you just reconstituted.
- Not labeling the date. You will not remember. Nobody remembers. Write it on the vial the moment you puncture it.
- Storing it in the bathroom. Humidity swings, temperature swings, steam from the shower. Main compartment of the fridge. Not the door (too warm every time you open it). Not the back wall (things freeze back there).
Frequently asked questions about bacteriostatic water
Sterile water + 0.9% benzyl alcohol. The alcohol stops bacteria from growing, so you can puncture the same vial for 28 days without it going bad. It is what you use to reconstitute freeze-dried compounds before injection.
No. Sterile water has no preservative. One puncture, then discard. Bac water has the benzyl alcohol that lets you use the same vial for 28 days. If you are drawing from a vial more than once (you are), use bac water.
Before you open it: no. Room temp is fine. After you put a needle through the stopper: yes, fridge, 2-8°C, 28 days max.
Unopened: until the manufacturer expiration date (typically 2-3 years from manufacture). After first puncture: 28 days when stored refrigerated at 2-8°C. This is the USP multi-dose vial standard. After 28 days, discard even if there is water remaining.
It depends on the compound and your target concentration. The most common reconstitution is 2 mL per 5 mg vial (giving 2.5 mg/mL). The amount of water does not change the total dose, only the concentration. More water = more dilute = easier to measure small doses. See the dosing table above or use the calculator for exact syringe units.
USP says discard. The benzyl alcohol itself is fine on day 29. The issue is every needle puncture introduced a tiny amount of contamination, and after 28 days of that, the math stops working in your favor. If it looks cloudy or smells off, discard immediately regardless of the date.
In the US, the FDA classifies it as a prescription product. In practice, it is widely available online without a prescription from medical supply retailers. Pharmacy enforcement varies by state and pharmacist. Outside the US, availability varies by country.
You can. You should not. A 30 mL vial costs under $10. Homemade versions risk wrong preservative concentration (too little and bacteria grow, too much and you damage tissue), incomplete sterilization, and particulate contamination. The risk-reward math does not work here.
Yes. Not a controlled substance. TSA allows medically necessary liquids exempt from the 3.4 oz rule. Keep it in carry-on, not checked (cargo holds get cold enough to freeze things). A pharmacy label helps if someone asks, but it is not required. See traveling with peptides for international rules.
No hard limit. The 28-day window is what caps it in practice. Most 30 mL vials get punctured 10-30 times. Fresh needle every draw. If rubber starts breaking off into the liquid (stopper coring), the vial is done regardless of the date.
No.
30 mL. Always. One vial reconstitutes 10-15 compound vials at 2 mL each. If you are running one compound, a single bac water vial outlasts the 28-day window easily. Running multiple compounds, you might need two open at once.
No. Benzyl alcohol is toxic to newborns. Use preservative-free sterile water for anything involving neonates. This does not apply to adults.
More water = more dilute, so you draw a larger volume for the same dose. Less water = more concentrated, so you draw less. Neither changes the total amount of compound in the vial. The only risk is measurement accuracy: too concentrated and small dose differences become hard to measure with a standard insulin syringe. The calculator shows exact syringe units for any reconstitution volume.
They are the same thing. USP bacteriostatic water contains 0.9% w/v (weight per volume) benzyl alcohol, meaning 9 mg per 1 mL. Some vendors list it as 0.9% and others as 0.9% w/v. Both refer to the same concentration. If a product lists a different percentage, it is not standard USP bacteriostatic water.
Where to go from here
÷ Open the peptide calculator Vial strength, bac water volume, dose. It tells you the exact syringe units.Also: bac water in Bangkok · reconstitution FAQ · storage FAQ · traveling with peptides · how to read a COA · something feels off?
Reviewed against USP standards and manufacturer documentation. Sep 2026. Not medical advice. Talk to a clinician before starting any protocol.
Key references
- USP Multi-Dose Container Standards: United States Pharmacopeia, General Chapter <797>
- Benzyl alcohol bacteriostatic mechanism: Nair, Int J Pharm 1983
- Gasping syndrome in neonates: Gershanik et al., N Engl J Med 1982
- CDC Multi-Dose Vial Policy: CDC Injection Safety FAQ
- FDA Bacteriostatic Water Drug Label: DailyMed / NLM