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The 28-day rule for bacteriostatic water, explained

It's a limit on how long a pierced multi-dose vial can be used. It isn't printed on the bac water label, and it says nothing about the peptide you mix into it.

By PeptideTracker editorial team · Updated November 8, 2026 · 7 sources

[00] Short answer

  1. Once a multi-dose vial is opened, CDC says to date it and throw it away within 28 days, unless the maker states another date3.
  2. USP <797>, the US pharmacy standard, sets the same limit: a multiple-dose container must not be used more than 28 days after first puncture2.
  3. The Hospira bac water label doesn't mention 28 days. It says the vial allows repeated withdrawals and has an expiry date1.
  4. The 28 days is about contamination of the vial, not how long a peptide stays intact. Approved products mixed with bac water range from 7 to 30 days4–7.
  5. Unopened bac water lasts until the expiry date on the vial, stored at 20-25 °C (68-77 °F)1,3.

Where does the 28-day rule for bacteriostatic water come from?

The 28-day rule comes from US infection-control and pharmacy standards, not from the bac water label. CDC says that once a multi-dose vial is opened (for example, needle-punctured), it should be dated and thrown away within 28 days unless the manufacturer states another date for the opened vial3. USP <797> sets the same 28-day limit for multiple-dose containers in pharmacies2.

Bacteriostatic water is a multi-dose vial. The Hospira label describes it as sterile water for injection with 0.9% (9 mg/mL) or 1.1% (11 mg/mL) benzyl alcohol, supplied in a multiple-dose container "from which repeated withdrawals may be made"1. So the 28-day rule applies to it once the stopper has been pierced.

What does the bac water label actually say?

The Hospira Bacteriostatic Water for Injection label gives no number of days after opening. It gives a storage temperature, 20-25 °C (68-77 °F), and an expiry date on each vial1. A few lines are worth knowing:

  • "Use aseptic technique for single or multiple entry and withdrawal from all containers."1
  • "Do not store reconstituted solutions of drugs for injection unless otherwise directed by the manufacturer of the solute."1 In other words, the medicine you mix in sets its own storage time.
  • "Do not use unless the solution is clear and seal intact."1
  • It must not be used in newborns, because benzyl alcohol has been linked to toxicity in neonates1.

Where an approved product comes with its own bac water, its instructions can name the 28 days directly. The EGRIFTA WR instructions say to throw away any bac water left in the bottle 28 days after first use, and to write down the date of first use4.

What USP <797> says about multi-dose vials

USP <797> says a multiple-dose container must not be used for longer than its beyond-use date or 28 days after it was first entered, whichever is shorter2. USP's FAQs add some detail:

  • 28 days is an in-use limit, not a shelf life. USP's example: a multiple-dose vial with a 60-day beyond-use date must still be thrown away no later than 28 days after first puncture2.
  • "Multiple-dose" has a definition. It means a container for injection that has met antimicrobial effectiveness testing, or is excluded from it by FDA regulation2. Bac water is labeled as a multiple-dose vial1. A peptide vial you mix at home hasn't been tested this way.
  • Labels win. Mixing a product according to its approved labeling isn't compounding under <797>, and in that case "the labeling must be followed"2.

<797> is written for pharmacies, and CDC's guidance is written for healthcare settings2–3. Neither was written for home use, but they're the source of the 28-day number people use at home.

Does the 28-day rule mean a mixed peptide lasts 28 days?

No. The 28 days limits how long a pierced vial can be used because of contamination risk. It doesn't test whether the medicine inside is still at full strength. Approved products mixed with bacteriostatic water show how much the answer depends on what's in the vial:

In-use limits after mixing with bacteriostatic water, from each product's US label
What's in the vialHow long the label allowsSource
The opened bac water vial itselfDate it and throw it away within 28 days, unless the maker gives another date2–3CDC, USP <797>
EGRIFTA WR (tesamorelin) mixed in that waterThrow away 7 days after mixing, kept at 20-25 °C. The bac water bottle itself: 28 days after first use4EGRIFTA WR label
Omnitrope (somatropin) vial, mixed with its own bac water diluentUse within 21 days, in the fridge at 2-8 °C7Omnitrope label
Ogivri (trastuzumab-dkst) 420 mg vial, mixed with bac waterFridge, 2-8 °C. Throw away after 28 days6Ogivri label
Novarel (chorionic gonadotropin), mixed with bac waterRefrigerate and use within 30 days5Novarel label
A research peptide mixed with bac waterNo tested limitNo label exists

EGRIFTA WR makes the point on its own: the bac water bottle can be used for 28 days, but the peptide mixed from it is thrown away after 74. Ogivri also shows the role of the preservative. Mixed with bac water it keeps 28 days in the fridge. Mixed with plain sterile water, the label says use it at once6.

For research peptides there's no label and no tested number. See how long reconstituted peptides last for what the general research says.

How long does unopened bacteriostatic water last?

Until the expiry date printed on the vial, if it's stored at 20-25 °C (68-77 °F) as the label says1. CDC says an unopened multi-dose vial should be thrown away according to the manufacturer's expiration date3.

  • The Hospira label gives no shelf life in months. The date on the vial is the limit1.
  • Once opened, the 28-day date can't run past the original expiry. CDC: "The beyond-use-date should never exceed the manufacturer's original expiration date."3
  • The label gives one storage temperature for the vial: 20-25 °C, which is controlled room temperature1.
[Tool]Plan how much bac water to addPuts your dose on a round syringe line

For how much water to add to a vial, see how much bac water to add.

Questions

Is the 28-day rule on the bacteriostatic water label?

Not on the Hospira label. It comes from CDC injection safety guidance and USP <797>, which say an opened multi-dose vial is dated and thrown away within 28 days unless the maker gives another date. Some kits, like EGRIFTA WR, repeat it in their own instructions.

Can I use bacteriostatic water after 28 days?

CDC and USP say no for an opened vial, unless the maker states a different date. The limit is about contamination of a vial that has been pierced, so it applies however much water is left.

Does my peptide last 28 days if I mix it with bacteriostatic water?

Not necessarily. The 28 days is for the opened bac water vial. Approved products mixed with bac water are given 7 days (EGRIFTA WR), 21 days (Omnitrope), 28 days (Ogivri) or 30 days (Novarel) by their labels. Research peptides have no tested limit.

Does bacteriostatic water need to be refrigerated after opening?

The Hospira label gives one storage temperature, 20-25 °C (68-77 °F), and doesn't ask for refrigeration after opening. The medicine you mix into it may need the fridge. Follow that medicine's label.

What's the difference between bacteriostatic water and sterile water?

Bacteriostatic water contains 0.9% or 1.1% benzyl alcohol as a preservative, so a vial can be entered more than once. Sterile water has no preservative. The Ogivri label shows the difference: mixed with bac water it keeps 28 days in the fridge, mixed with sterile water it must be used at once.

Sources

Checked against Bacteriostatic Water for Injection, USP (Hospira) label on DailyMed (2026), USP <797> FAQs (Dec 2023), CDC injection safety guidance, and the EGRIFTA WR, Novarel, Ogivri and Omnitrope US labels. How we check

  1. 1Bacteriostatic Water for Injection, USP (Hospira), prescribing information (DailyMed). description, warnings, precautions, how supplied; version effective May 2026
  2. 2USP <797> Pharmaceutical Compounding: Sterile Preparations, FAQs (updated 11 Dec 2023). FAQs 178, 187, 188, 193 and 194
  3. 3CDC: Preventing Unsafe Injection Practices. multi-dose vials; page dated 26 Mar 2024
  4. 4EGRIFTA WR (tesamorelin for injection), US prescribing information, Patient Information and Instructions for Use (DailyMed). sections 2.2 and 16; version effective July 2026
  5. 5Novarel (chorionic gonadotropin for injection), US prescribing information (DailyMed). description; version effective August 2025
  6. 6Ogivri (trastuzumab-dkst), US prescribing information (DailyMed). section 2, reconstitution; version effective June 2026
  7. 7Omnitrope (somatropin), US prescribing information (DailyMed). sections 11 and 16.2; version effective July 2025

This guide explains what labels and published studies say. It isn't medical advice and doesn't replace your prescriber or pharmacist.

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