Single-dose and multi-dose vials
The CDC draws a clear line. A single-dose vial is approved for one patient and one injection or procedure. It usually has no antimicrobial preservative, so leftover medicine is not kept for later, even for the same person, and leftovers from several vials are never pooled [1].
A multi-dose vial holds more than one dose and is approved for repeated use. Its manufacturer labels it as such, and it typically contains an antimicrobial preservative to limit the growth of bacteria [1]. The labels show the difference in practice: the multi-dose Zepbound® vial lists benzyl alcohol and phenol among its ingredients, while the single-dose vial and pen do not, and the single-dose Gattex® vial states that no preservatives are present [3][10].
What a preservative can and cannot do
A preservative is not a guarantee. The CDC states that it has no effect on viruses and does not give complete protection against bacterial contamination [1]. Each time a needle passes through the rubber stopper, there is a chance of bringing something in.
That is why time matters. The FDA explains its own advice on multi-dose vials by noting that the risk of contamination rises after 28 days [2]. The label of a preserved diluent makes the same point from the other side: bacteriostatic water is sold in a multiple-dose container, yet its label still asks for aseptic technique at every entry [11].
Where the 28 days come from
The figure comes from USP chapter 797, the US Pharmacopeia standard for sterile preparations, as summarised by the CDC. An unopened multi-dose vial is kept until the manufacturer’s expiry date. From the first needle puncture, the vial is dated and has at most 28 days of use left, unless the manufacturer sets a different period for the opened vial [1].
One more limit applies: the discard date, which USP calls the beyond-use date, can never be later than the manufacturer’s original expiry date [1]. If a vial expires in two weeks, two weeks is the limit.
What the FDA adds for compounded vials
The FDA has addressed this directly for compounded GLP-1 medicines, which are often dispensed in multi-dose vials for people to use at home. It generally advises using such vials within 28 days of first use and then discarding them, even if medicine is left [2].
The FDA is explicit that this applies even when the compounder’s instructions say the medicine can be used for longer than 28 days. It also asks people to use a new sterile needle and syringe for every dose, because reusing them can contaminate the medicine [2].
When the label gives its own date
The 28 days are a default for when the manufacturer is silent. When an approved label sets an in-use period, that period applies, and it may be shorter or longer. Pens are included below because several of them also hold more than one dose, even though they are not vials.
| Product and presentation | Limit after first use or mixing | Source |
|---|---|---|
| Zepbound® or Mounjaro® multi-dose vial or single-patient-use KwikPen® | 30 days after first use, 30 days in total at room temperature or 4 weekly doses, whichever comes first, even with medicine left | Labels, section 16.2 [3][4] |
| Ozempic® single-patient-use pen | 56 days after first use, at room temperature or in the fridge | Label, section 16.2 [5] |
| Wegovy® FlexTouch® single-patient-use pen | Up to 56 days after first use | Label, section 16.2 [6] |
| Saxenda® or Victoza® pen | 30 days after first use | Labels, section 16.2 [7][8] |
| Egrifta WR™ vial mixed with its diluent | 7 days after mixing; the bacteriostatic water bottle itself 28 days after first use | Label, section 2.2 and Instructions for Use [9] |
| Gattex® single-dose vial | Mixed solution used within 3 hours, the rest discarded | Label, section 2.6 [10] |
| Compounded multi-dose vial | No FDA-reviewed period; the FDA advises 28 days after first use | FDA [2] |
Handling habits from the CDC, FDA and MedlinePlus
- Use a new sterile needle and syringe for every entry into the vial [1][2].
- Wipe the rubber top with an alcohol pad before drawing, as MedlinePlus describes [12].
- Never leave a needle sitting in the stopper between doses; the CDC says it gives microorganisms a direct route into the liquid [1].
- Keep a multi-dose vial for one person, and never share a vial, a syringe or a pen, even with a new needle [1][3].
- Look at the liquid each time: MedlinePlus lists a change in colour, floating pieces and cloudiness as signs to stop [12].
- Prepare each injection as close as possible to the time you give it [1].
- Throw a vial away whenever its sterility is compromised or in doubt [1].
Counting the days
Day 1 is the day of the first puncture, not the day you received the vial. The CDC asks for the vial to be dated when it is opened, and MedlinePlus gives the same tip for any vial you will use more than once [1][12]. Writing the date on the label or the box is the simplest step.
Then compare the dates that apply and use the earliest: the label’s in-use period if it has one, otherwise 28 days after the first puncture, and in every case the expiry date printed on the vial [1]. A vial used weekly, or for small doses, can easily outlast its period without anyone noticing; the table above shows how much the periods differ between products.
What a hospital survey found
How well do people keep to this? Evidence from home use is thin, but a 2004 prevalence study gives a picture from a hospital. It was prompted by two deaths from meningitis in Germany traced to a contrast medium used as a multi-dose vial over 8 days. On one day in November 2001, infection control nurses in a large German hospital collected every opened multi-dose vial in use, 227 in all [13].
One vial and one dispensing spike were contaminated with Staphylococcus epidermidis, a rate of 0.9%. The opening date was marked on only half of the vials, and 13% of those dated had already expired [13]. It is one hospital on one day, more than two decades ago, so it cannot say how often home vials are contaminated; its authors drew practical lessons that match the guidance above: hand hygiene, disinfecting the stopper, following the manufacturer and marking the opening time [13].
Ozempic®, Wegovy®, FlexTouch®, Saxenda® and Victoza® are registered trademarks of Novo Nordisk A/S. Zepbound®, Mounjaro® and KwikPen® are registered trademarks of Eli Lilly and Company. Gattex® is a registered trademark of Takeda Pharmaceuticals U.S.A., Inc. Egrifta WR™ is a trademark of Theratechnologies Inc. PepLabo is not affiliated with or endorsed by Novo Nordisk, Eli Lilly and Company, Takeda or Theratechnologies.
When to get help
| Sign | Who to contact | How fast | Source |
|---|---|---|---|
| Redness, swelling, red streaks or fever after an injection | Your prescriber or healthcare provider | Today | MedlinePlus, subcutaneous injections [14] |
| The liquid looks cloudy, discoloured or has floating particles | Do not use it; ask your pharmacist | Today | MedlinePlus [12]; Zepbound® label 2.4 [3] |
| A needle was left in the stopper, or the vial was entered with a used needle | Treat its sterility as in doubt and ask your pharmacist before the next dose | Today | CDC [1] |
| The compounder’s instructions allow longer than 28 days | Your pharmacist or prescriber; the FDA advises 28 days | At your next appointment | FDA [2] |
