Read the drug's label, not the water'sBac water's label says it is "indicated only for diluting or dissolving drugs for intravenous, intramuscular or subcutaneous injection, according to instructions of the manufacturer of the drug to be administered."[1] Whether a peptide may be mixed with it, and for how long, is the drug's label's call. Most peptides sold online have none.
What bac water is, from its FDA label
Bac water is a sterile, preserved water made to be drawn from more than once; these are the facts on its current label.[1]
- Composition
- Water for injection with 0.9% (9 mg/mL) or 1.1% (11 mg/mL) benzyl alcohol as a bacteriostatic preservative
- pH
- 5.7 (range 4.5–7.0)
- Containers
- 30 mL plastic vial (9 mg/mL) or 20 mL glass vial (11 mg/mL); multiple-dose
- Status
- Prescription only; store at 20–25 °C
- Boxed warning
- "NOT FOR USE IN NEONATES"; for newborns, only preservative-free Sterile Water for Injection should be used
- Before use
- Must be made approximately isotonic; given intravenously without a dissolved drug it can cause hemolysis
- After opening
- No in-use period is stated. The label says not to store reconstituted solutions "unless otherwise directed by the manufacturer of the solute"
How long does bac water last after opening?
Its own label sets no limit, but for healthcare settings the CDC says an opened multiple-dose vial is discarded within 28 days unless its manufacturer says otherwise.[2]
The rule in full. The bac water label sets no limit, but for multiple-dose vials in healthcare settings the CDC, citing the USP 797 standard, says an opened vial "should be dated and discarded within 28 days" unless the manufacturer states another date.[2] That rule is about the water vial itself. It is not a shelf life for a peptide dissolved in it; see how to store peptides.
Bac water vs sterile water for peptides
The difference is the preservative: bac water has one and is multiple-dose, sterile water has none and is used once. Both appear on current tesamorelin labels.
| Bacteriostatic Water for Injection | Sterile Water for Injection | |
|---|---|---|
| Preservative | Benzyl alcohol | None |
| Containers | Multiple-dose | Single-use bottles in the Egrifta SV pack |
| Approved peptide that uses it | Egrifta WR: 1.3 mL per 11.6 mg vial[3] | Egrifta SV: 0.5 mL per 2 mg vial[4] |
| After mixing (that product) | Discard 7 days after mixing | Use immediately; discard leftovers |
| Newborns | Not for use | The water to use instead |
The same peptide, tesamorelin, comes with either water depending on the product, and the in-use limits differ with it. That is why "can I use bac water" has no general answer.
How much bac water to add: the arithmetic
The volume of bac water sets the concentration; it never changes how much peptide is in the vial. This table gives the volume that makes one U-100 unit hold a round number of micrograms.[5]
| Vial | 1 unit = 10 mcg | 1 unit = 25 mcg | 1 unit = 50 mcg | 1 unit = 100 mcg |
|---|---|---|---|---|
| 2 mg | 2 mL | 0.8 mL | under 0.5 mL | under 0.5 mL |
| 5 mg | 5 mL | 2 mL | 1 mL | 0.5 mL |
| 10 mg | over 5 mL | 4 mL | 2 mL | 1 mL |
Check one row: 5 mg with 2 mL is 2.5 mg/mL, so each unit (0.01 mL) holds 25 mcg. The calculator above shows the same for any volume, and the reconstitution calculator works backwards from a target reading. For approved products, the label's volume is the only one that applies.
What the FDA says about bac water sold with peptides
The FDA treats bac water sold alongside research peptides as part of the evidence that the peptides are meant for people, and in some letters as a drug in its own right.
- Gram Peptides (March 2026): "The sale of these products together demonstrates that you intend for your 'Bacteriostatic Water for Injection' to be used in combination for injection. Therefore, your 'Bacteriostatic Water for Injection' is a drug."[6]
- Royal Peptides and NuScience Peptides (August 2026): bac water marketed alongside a "peptide calculator" was cited as providing "the means to prepare an injectable drug for human administration".[7][8]
- Peak Performance Peptides and others (August 2026): "you market bacteriostatic water as a solvent for reconstituting your products".[9]
- USApeptide.com (February 2025): bac water and packs of insulin syringes offered with injectable products.[10]
This site sells nothing, including water. The safety page covers these letters in more detail.
Related pages
- How to store peptidesRoom temperature, fridge and in-use limits from the labels
- How to reconstitute peptidesThe five numbers, in order
- Tesamorelin calculatorEgrifta SV and WR label math
Frequently asked questions
What is bac water for peptides?
Bacteriostatic Water for Injection, USP: sterile water with 0.9% benzyl alcohol, packaged for repeated withdrawals. Its label limits it to dissolving drugs as each drug's manufacturer directs.[1]
How much bac water do I add to a 5 mg peptide vial?
The arithmetic gives options: 1 mL makes each unit hold 50 mcg, 2 mL makes it 25 mcg, and 2.5 mL makes it 20 mcg. For an approved product, the label sets the volume. Which liquid is appropriate, if any, is a question for a pharmacist.
How long does bac water last after opening?
Sources
- Bacteriostatic Water for Injection, USP (Hospira) label
- Preventing unsafe injection practices (multi-dose vials, citing USP 797)
- Egrifta WR (tesamorelin) prescribing information
- Egrifta SV (tesamorelin) prescribing information
- 510(k) clearance K233794, Insulin Syringe ("U-100 means there are 100 units of insulin in every mL")
- Warning letter: Gram Peptides
- Warning letter: Royal Peptides LLC
- Warning letter: NuScience Peptides LLC
- Warning letter: Peak Performance Peptides
- Warning letter: USApeptide.com