Bac water has two clocks. The first is the expiry date on the label, which applies while the vial is sealed. The second starts the first time a needle goes through the stopper.
| Situation | How long it lasts | Source |
|---|---|---|
| Sealed, unopened vial | Until the expiry date on the label | The vial’s label |
| After the first puncture | 28 days, unless the manufacturer specifies otherwise, and never past the printed expiry | USP ⟨797⟩; CDC injection-safety guidance |
| Mixed with a peptide | As the instructions for that peptide state | The product being prepared |
Unopened bacteriostatic water
A sealed vial is good until the expiry date printed on it, provided it has been kept in the conditions on its label. That date is set by the manufacturer for the closed container. It says nothing about how long the vial stays usable once it has been entered.
How long bac water lasts after opening
Once a multiple-dose vial has been punctured, USP chapter ⟨797⟩ sets its beyond-use date at 28 days, unless the manufacturer specifies a different period. CDC injection-safety guidance gives the same 28 days. The date comes from the preservative: benzyl alcohol limits bacterial growth after entry, but not indefinitely. The 28 days never runs past the expiry date on the label, so a vial opened a week before it expires has a week, not 28 days.
This is why vials are commonly marked with the date of first entry: it is the date the 28 days count from.
Does bac water need to be refrigerated?
The vial itself, no. The label for Bacteriostatic Water for Injection, USP gives its storage as 20 to 25 °C (68 to 77 °F), which is controlled room temperature. Refrigeration is not part of its labelled storage.
Refrigeration questions usually come from the peptide rather than the water. Many reconstituted peptides are stored cold, and that requirement belongs to the peptide’s own instructions. Once the water has been used to prepare a product, the product’s storage conditions and time limits are the ones that apply, and they can be shorter than the water’s 28 days.
Bacteriostatic water storage, in brief
- Labelled storage for the vial: 20 to 25 °C (68 to 77 °F).
- Expiry: the date printed on the label, for a sealed vial.
- After first puncture: 28 days unless the manufacturer says otherwise, and never beyond the expiry date.
- After mixing: the instructions for the prepared product.
Signs a vial is past use
A vial is treated as unusable when it is past its expiry or its 28 days, when it has been stored outside its labelled conditions, when the seal or stopper is damaged, or when the liquid shows particles or discoloration. Appearance alone does not prove a vial is fine: clear liquid can still be contaminated, which is why the dates matter more than the look.
Why vial size and shelf life are linked
The 28-day limit is one reason vial sizes exist. A 30 mL bottle costs less per mL than a 10 mL vial, but only the volume used within 28 days of opening counts. Smaller vials suit occasional use; larger ones suit steady use where the volume will be drawn down within the period.
Sources: DailyMed, Bacteriostatic Water for Injection, USP; USP ⟨797⟩ Pharmaceutical Compounding, Sterile Preparations; CDC, Preventing Unsafe Injection Practices.