Bacteriostatic Water for Peptides: What It Is, How Much to Add
Bacteriostatic water is the liquid most people use to turn a freeze-dried peptide into something they can inject. It is not magic and it is not interchangeable with sterile water. The volume you add is not a medical parameter either: it only sets the concentration, and so the number of units you draw.
- Bacteriostatic Water for Injection, USP is sterile water with 0.9% benzyl alcohol (9 mg/mL) as a preservative. The label calls it a multiple-dose vial and allows repeated withdrawals.
- Sterile Water for Injection has no preservative and comes only in single-dose containers: the label says to discard the unused portion.
- How much water you add does not change how much peptide is in the vial. It changes the concentration, and therefore how many units on a U-100 syringe equal your dose.
- The '28-day' rule comes from CDC guidance for opened multi-dose vials, and it gives way to the manufacturer's date. Approved peptide labels set their own limits, from 'use immediately' to 30 days.
Bacteriostatic water (often shortened to BAC water) is sterile water for injection that contains 0.9% benzyl alcohol, 9 mg per mL, as a bacteriostatic preservative1. The preservative lets you puncture the vial more than once. The FDA-listed label calls it a multiple-dose vial and says repeated withdrawals may be made1. There is no correct amount to add to a peptide vial in general. The volume only decides the concentration, and so how many syringe units make up a given dose. More water means a more dilute solution and more units per dose, and less water means the opposite. The peptide itself stays the same. The rest of this guide covers what the label says, how BAC water differs from sterile water, where the '28 days' figure comes from, and how to do the volume math. The step-by-step mixing and injection technique is in our guide on how to inject peptides, so it is not repeated here.
What bacteriostatic water is, according to its label
The Hospira label for Bacteriostatic Water for Injection, USP lists two strengths of benzyl alcohol, 0.9% (9 mg/mL) and 1.1% (11 mg/mL). It gives a pH of 5.7 (range 4.5 to 7.0) and says to store the vials at 20 to 25°C (68 to 77°F)1. The label's only indication is 'for diluting or dissolving drugs for intravenous, intramuscular or subcutaneous injection, according to instructions of the manufacturer of the drug to be administered'1. The last clause matters most. BAC water is a vehicle, and the drug's own instructions decide whether it is the right one and how much to use.
The label carries two warnings. First, it is not for use in neonates. Benzyl alcohol has been associated with toxicity in newborns, and the label directs that only preservative-free sterile water be used for them1. The problem was first described in the early 1980s as the 'gasping syndrome' in premature infants4, and approved drugs supplied with BAC water still repeat the warning5. For adults, the label cites experimental data estimating that up to 30 mL could be given intravenously without toxic effects1. That is far more than the fraction of a millilitre used in a subcutaneous dose. Second, water with no solute injected into a vein can cause hemolysis, which is why the label ties intravenous use to an approximately isotonic mixture1.
The label also says what BAC water does not do: 'Do not store reconstituted solutions of drugs for injection unless otherwise directed by the manufacturer'1. The preservative protects the water vial against contamination from repeated needle entry. It does not promise that a peptide dissolved in it stays potent for any set length of time.
Bacteriostatic water vs sterile water
Sterile Water for Injection, USP is the same base liquid with nothing added. Its label describes a sterile, nonpyrogenic water that 'contains no bacteriostat, antimicrobial agent or added buffer and is supplied only in single-dose containers'2. It also says: 'Do not reuse single-dose containers. Discard unused portion'2. Without a preservative, nothing slows the growth of bacteria that get in with the first needle. That is why the vial is meant to be entered once.
| Bacteriostatic Water for Injection, USP | Sterile Water for Injection, USP | |
|---|---|---|
| Preservative | Benzyl alcohol 0.9% (9 mg/mL) or 1.1%1 | None: 'no bacteriostat, antimicrobial agent or added buffer'2 |
| Container type | Multiple-dose vial; repeated withdrawals allowed1 | Single-dose containers only2 |
| After first use | Keep for repeated withdrawals; no discard date on the label1 | 'Do not reuse single-dose containers. Discard unused portion'2 |
| Neonates | Not for use in neonates1 | The preservative-free option the BAC label points to for neonates1 |
| Storage of the unopened vial | 20–25°C (68–77°F)1 | 20–25°C (68–77°F)2 |
| When a drug label names it | Use it only if the drug's instructions allow it | Use it if the drug's instructions require it |
Neither water is better in general. The drug decides. Two FDA-approved versions of the same peptide, tesamorelin, show this. EGRIFTA SV comes with a 10 mL single-dose bottle of Sterile Water for Injection. Its label says to inject the dose right after mixing, throw away any unused solution and diluent, and not freeze or refrigerate the reconstituted solution7. EGRIFTA WR comes with a 30 mL multiple-dose bottle of Bacteriostatic Water for Injection. Its label says to use only that diluent, and the mixed vial provides daily doses for seven days6. The label also states that the two products are not substitutable6. The formulation and the diluent were designed and tested together.
Where the '28 days' figure comes from
The 28-day number that circulates for BAC water comes from infection-control guidance for multi-dose vials in healthcare. The CDC says: 'Once a multi-dose vial is opened (e.g., needle-punctured) the vial should be dated and discarded within 28 days unless the manufacturer states another date for that opened vial'3. The same guidance adds that this beyond-use date should never go past the manufacturer's original expiration date3.
Two things follow. First, the rule covers the opened vial as a container. It is a limit on contamination risk, not a stability result for any drug dissolved from it. Second, it gives way to the manufacturer. For approved peptides, the product label sets the time limit for the mixed drug, and those limits vary widely:
| Product (FDA label) | Diluent | Limit after mixing |
|---|---|---|
| EGRIFTA SV (tesamorelin) | Sterile Water for Injection, single-dose7 | Use immediately; discard any unused solution; do not refrigerate or freeze7 |
| EGRIFTA WR (tesamorelin) | Bacteriostatic Water for Injection, 30 mL multiple-dose6 | Room temperature 20–25°C; discard 7 days after mixing; do not freeze6 |
| NOVAREL (chorionic gonadotropin) | Bacteriostatic Water with 0.9% benzyl alcohol, 10 mL5 | Refrigerate at 2–8°C and use within 30 days5 |
| GENOTROPIN 5 mg / 12 mg cartridge (somatropin, with preservative) | Built-in diluent chamber8 | Refrigerate between uses; discard after 28 days8 |
| GENOTROPIN MINIQUICK (somatropin, without preservative) | Built-in diluent chamber8 | Up to 24 hours refrigerated; single use8 |
The same molecule (somatropin) can have a 28-day window with a preservative and a 24-hour window without one8. That is the clearest sign that the preservative and the formulation decide the number. The diluent's name alone does not. A peptide sold without FDA review has no tested number of its own. The page on whether peptides are safe explains why stability and purity claims for those products can't be checked.
How much bacteriostatic water to add
When a drug has a label, the answer is on it. EGRIFTA WR says to add 1.3 mL of the supplied bacteriostatic water to each vial, which gives 8 mg per mL6. The BAC water label defers in the same way: the volume 'is dependent on the vehicle concentration, dose and route of administration as recommended by the manufacturer'1.
One label shows how volume is a free choice with fixed math. NOVAREL lets the user add either 1 mL or 10 mL of bacteriostatic water. A 5,000-unit vial becomes 5,000 units per mL with 1 mL, meant for giving the whole dose at once, or 500 units per mL with 10 mL, meant for multiple doses stored in the fridge between injections5. The vial holds the same drug either way. Only the concentration changes, and so the volume each dose takes.
For a vial with no label, the same logic gives three practical limits on the choice:
- The syringe has to measure the dose. A 1 mL U-100 insulin syringe holds 100 units, and the smaller 0.3 and 0.5 mL barrels hold 30 and 50. If the dose works out to 2 or 3 units, one mark of error is a large share of the dose. Adding more water spreads the same dose over more units. Our guide to insulin syringe units covers barrel sizes and markings.
- The dose volume has to stay small. Subcutaneous doses are usually a fraction of a millilitre. Diluting until one dose fills most of a 1 mL syringe makes each injection larger than it needs to be.
- The vial has to fit the water. A small peptide vial holds only a few millilitres. Pick a volume that leaves room and gives round numbers you can repeat.
How the volume changes the units you draw
The math is the same for every peptide. Concentration = amount of peptide in the vial ÷ volume of water added. Dose volume = dose ÷ concentration. On a U-100 syringe, 1 mL = 100 units, so units = dose volume in mL × 100. Here is the same 250 mcg dose from a vial containing 5 mg (5,000 mcg), mixed three ways:
| Water added to a 5 mg vial | Concentration | Volume for 250 mcg | Units on a U-100 syringe |
|---|---|---|---|
| 1 mL | 5,000 mcg/mL | 0.05 mL | 5 units |
| 2 mL | 2,500 mcg/mL | 0.10 mL | 10 units |
| 2.5 mL | 2,000 mcg/mL | 0.125 mL | 12.5 units |
| 3 mL | ≈1,667 mcg/mL | 0.15 mL | 15 units |
All four rows deliver the same 250 mcg. The 2 mL row gives the easiest number to read on the syringe. The 2.5 mL row gives a half unit, which most 1 mL barrels can't mark. The worked numbers are arithmetic, not a dose recommendation. Every vial size and water volume gives a different table. Our peptide reconstitution calculator does the division for any vial, volume and dose, and the reconstitution chart lays out the common combinations side by side.
Common mistakes with bacteriostatic water
| Mistake | Why it matters | What the labels say instead |
|---|---|---|
| Using one sterile water vial for several draws | Nothing in it limits bacterial growth after the first needle | 'Do not reuse single-dose containers. Discard unused portion'2 |
| Using BAC water where a drug requires sterile water | The preservative is part of what was tested; a different diluent is an untested formulation | EGRIFTA SV ships with sterile water, and WR's label says to use only its own diluent; the two are not substitutable6,7 |
| Treating '28 days' as the life of the dissolved peptide | The CDC figure is a container rule and gives way to the manufacturer3 | Approved labels range from immediate use to 30 days5,6,7,8 |
| Changing the water volume without redoing the math | The same number of units now holds a different dose | Concentration follows the volume: 5,000 vs 500 units/mL from one vial5 |
| Shaking the vial to dissolve the powder | Approved labels for reconstituted peptides tell you to swirl instead | EGRIFTA WR: 'Move the vial in a circle (swirl)… Do not shake'6 |
| Using BAC water to prepare a medicine for a newborn | Benzyl alcohol is linked to toxicity in neonates | 'NOT FOR USE IN NEONATES'1 |
Mixed peptide in a multi-dose product also contains the preservative, and approved multi-dose peptide products add their own. The multi-dose MOUNJARO vial and KwikPen, for example, contain benzyl alcohol (5.4 mg per dose) and phenol. The label still caps an opened vial or pen at 30 days, or at four weekly doses, even if medicine remains9. A preservative lets a vial be used more than once. The label still sets an end date.
This guide explains a diluent, not a protocol
Everything above comes from FDA-listed labels and CDC guidance on the waters and approved drugs named. None of it is a recommendation to use any peptide, or a dose. If you have a prescription, the drug's own label and your pharmacist decide the diluent, the volume and how long the mixed vial lasts. If you log doses in Peptide Me, enter the vial size and water volume you actually used. The app then shows each dose in units from that concentration, so a change in volume doesn't leave you drawing the old number.
FAQ
References
Primary sources — PubMed / NEJM / The Lancet.
- DailyMed (U.S. National Library of Medicine). Bacteriostatic Water for Injection, USP (benzyl alcohol 0.9% and 1.1%) — label, Hospira, Inc. Revised 08/2019. https://dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=87d6e9dc-fe3b-4593-ac9a-d7493d1959c7
- DailyMed (U.S. National Library of Medicine). Sterile Water for Injection, USP — single-dose vial label, Hospira, Inc. Revised 03/2025. https://dailymed.nlm.nih.gov/dailymed/lookup.cfm?setid=3a6422e1-feee-4095-8883-2b0837039e6e
- Centers for Disease Control and Prevention. Preventing Unsafe Injection Practices. Injection Safety. https://www.cdc.gov/injection-safety/hcp/clinical-safety/index.html
- Gershanik J, Boecler B, Ensley H, McCloskey S, George W. The gasping syndrome and benzyl alcohol poisoning. N Engl J Med. 1982;307(22):1384-1388. https://pubmed.ncbi.nlm.nih.gov/7133084/
- DailyMed (U.S. National Library of Medicine). NOVAREL (chorionic gonadotropin) for injection — Prescribing Information, Ferring Pharmaceuticals. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=ac6918ff-9dc3-40d9-8619-54b26e4bdfc8
- DailyMed (U.S. National Library of Medicine). EGRIFTA WR (tesamorelin) for injection — Prescribing Information, Theratechnologies Inc. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=839334d3-8c1d-4c26-9036-2ab524a6ea75
- DailyMed (U.S. National Library of Medicine). EGRIFTA SV (tesamorelin) for injection — Prescribing Information, Theratechnologies Inc. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=3d783378-b02d-4f19-99dd-0fc91a042224
- DailyMed (U.S. National Library of Medicine). GENOTROPIN (somatropin) for injection — Prescribing Information, Pfizer. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=ffebf88b-d257-4542-9808-74d9b7167765
- DailyMed (U.S. National Library of Medicine). MOUNJARO (tirzepatide) injection — Prescribing Information, Eli Lilly and Company. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=d2d7da5d-ad07-4228-955f-cf7e355c8cc0