The 28-Day Rule: Why USP 797 Sets a Hard Limit on Opened Multi-Dose Vials
What USP 797 Actually States
United States Pharmacopeia General Chapter 797, as summarised by the CDC, says an opened multi-dose vial should be dated and discarded within 28 days unless the manufacturer states another date.
That means 28 is not a suggestion or a rough guide. It is the default ceiling written into a pharmacy compounding standard that laboratories and clinics reference for handling opened vials.
The rule applies from the moment the vial is first punctured, not from the date it was manufactured or the date it arrived at your bench. Write the puncture date on the vial the moment you open it.
The Manufacturer's Expiry Still Wins
The beyond-use date should never exceed the manufacturer's original expiry date printed on the vial. If a vial's printed expiry is 12 days away when you open it, your beyond-use date is 12 days, not 28.
This is a two-check system. First check the printed expiry. Second apply the 28-day window from first puncture. Whichever date comes first is the one that governs the vial.
Skipping this comparison is a common labelling error. A technician who only writes '28 days from today' on every vial, without glancing at the printed expiry, can end up using a vial well past its manufacturer date.
Why the Preservative Isn't a Free Pass
Multi-dose vials typically contain an antimicrobial preservative, and the CDC notes it does not act on viruses and does not give complete protection against bacterial contamination.
This is the practical reason the 28-day figure exists at all. The preservative slows bacterial growth after each puncture; it does not sterilise the vial again or reset a clock. Every entry into the septum is a fresh opportunity for contamination, and the preservative is a partial defence, not a seal.
Because the preservative has no effect on viral contamination, the 28-day limit is a chemistry and biology compromise, not an arbitrary bureaucratic figure. It reflects how long a preserved solution can be expected to resist bacterial proliferation under normal storage conditions before risk becomes unacceptable.
One Needle, One Syringe, Only One Time
The CDC's rule of thumb for safe injection practice is One Needle, One Syringe, Only One Time. This phrase exists because every needle insertion into a septum is a separate contamination event, regardless of the 28-day window.
The two rules work together. The 28-day rule limits how long a vial stays in service. The one-needle rule limits what happens each time you access it. A vial can be within its 28-day window and still be compromised if accessed with a reused needle or syringe.
In a lab setting, this means a fresh sterile needle and syringe for every single draw from a multi-dose vial, no exceptions, even if the same vial is used multiple times within its dated window.
Building the Habit Into Your Bench Setup
A simple system works better than memory. Label the vial with the puncture date the second it is opened, then calculate and write the earlier of the two dates: 28 days out or the printed expiry, whichever comes first.
Sydney Bacwater's vial cases include space for a written date on the lid or sleeve, which keeps the beyond-use date visible without relying on marking the glass itself.
Store opened vials according to the manufacturer's printed storage instructions between uses, and keep sterile needles and syringes in single-use packaging until the moment of the draw. This closes the gap between what USP 797 requires on paper and what actually happens at the bench.