Single-Dose vs Multi-Dose Vial
The single-dose and multi-dose distinction turns on whether a formulation contains a preservative validated for repeated entry, not on how much liquid the vial happens to hold.
A single-dose vial holds a formulation with no antimicrobial preservative and is labelled for one entry and one patient, with any remainder discarded. A multi-dose vial contains a preservative system, usually benzyl alcohol, metacresol or phenol, shown by preservative effectiveness testing to suppress organisms introduced at each puncture. The difference is a property of the formulation, verified in a specific container with a specific stopper and stated on the label. Vial size and remaining volume have nothing to do with it.
The dating rules follow. A punctured multi-dose vial carries a beyond-use date from first entry, conventionally twenty-eight days unless the manufacturer states otherwise, because preservative efficacy and stability were demonstrated over a defined in-use period rather than indefinitely. Single-dose vials have no such window at all. Public health campaigns exist because outbreaks of bloodborne infection have been traced to single-dose vials split across patients and multi-dose vials carried between them.
The tradeoff is waste and cost against contamination risk and excipient exposure. Multi-dose presentations lower cost per administration and suit high-volume settings, but preservatives are not inert: they are irritating, they are contraindicated in neonates and for intrathecal and intraocular routes, and several destabilise peptides, which is one reason many biologics are single-dose.
The characteristic misunderstanding in unregulated use is that adding a preserved diluent converts a vial into a multi-dose product. It does not. A preserved diluent contributes a preservative concentration never validated against that formulation, container or closure, and effectiveness depends on all three. The second error is assuming the in-use clock resets on refrigeration or at the next entry.