The label does not give a number, and there is no pharmacopeial limit on the number of entries. What limits a multi-use vial in practice is a combination of three things: the stopper’s ability to reseal, the accumulating risk of coring and contamination with each entry, and the 28-day in-use convention, which for most laboratories arrives long before the stopper is the problem.

What the stopper is tested for

Elastomeric closures for multi-use vials are tested under USP <381> for penetrability (the force needed to pass a transfer device through them), fragmentation (how many rubber particles a series of entries produces) and self-sealing (whether the closure reseals after repeated entries so that the contents stay contained and sterile). A bromobutyl stopper that passes those tests will tolerate many entries; the tests themselves use on the order of ten entries per closure. See vial stoppers explained.

Why fewer is still better

• Coring. Each entry can cut a small fragment from the stopper, and the probability rises with a blunt or reused transfer device, with entering the same spot, and with the number of entries. Fragments are visible particles and mean the vial is discarded. See particulate matter testing.
• Contamination. The preservative handles small numbers of organisms introduced occasionally; it is not designed for a vial entered dozens of times a day. Every entry is a chance to introduce organisms, and the preservative’s margin is finite. See what does bacteriostatic mean?
• Air exchange. Each withdrawal admits air, which accelerates oxidation of the benzyl alcohol. See benzyl alcohol oxidation.

A practical rule

Match the vial size to the number of withdrawals expected in 28 days and treat the calendar, not the stopper, as the limit. A laboratory drawing from a vial twice a day for four weeks makes about 50 entries, which a good stopper tolerates; one drawing from it twenty times a day is better served by several smaller vials opened in sequence. Wipe the stopper before every entry, use a fresh sterile transfer device each time, enter at different points on the stopper’s target area, and inspect the contents for fragments after the first few entries. See 70% vs. 99% isopropyl alcohol for wiping stoppers and shelf life after opening.

Related

Multi-use vs. single-use vials · How labs choose a vial size

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