Multi-dose vial (revision 23)

A multi-dose vial, termed a multiple-dose container in compendial language, is a container of an injectable preparation intended to permit withdrawal of successive portions of its contents without change in the strength, quality or purity of the remainder. The definition is functional: it describes an intention and the conditions that make the intention safe, not a size or a shape. A 10 mL vial may be a single-dose or a multiple-dose container depending on its formulation and its labelling.^[1]

Two requirements follow from repeated entry. Because each penetration of the closure is an opportunity for microbial ingress, and because the contents are held at or near room temperature for a period afterwards, a multiple-dose preparation must contain an antimicrobial preservative whose effectiveness has been demonstrated by the challenge testing described in USP <51>. And because the preservative controls but does not eliminate risk, the period during which the container may be used after its first puncture is limited, conventionally to 28 days unless the manufacturer specifies otherwise.^[2][3]

The 28-day convention is frequently misread as a statement about chemical stability. It is not: it derives from the antimicrobial effectiveness of preserved formulations and from infection-control practice, and it is independent of whether the active substance remains within specification. A preparation may be chemically stable for a year and still be discarded at 28 days, and a preparation may lose potency well before 28 days despite its preservative performing perfectly.^[3][4]

Documented outbreaks of blood-borne and bacterial infection traced to multiple-dose vials have been reported repeatedly, almost always from practices that reuse a syringe or needle to re-enter a vial rather than from failure of the preservative itself. This distinction — between the container as a vector and the container as a reservoir — shapes the infection-control guidance and explains why that guidance concentrates on the syringe.^[5][4]

USP <659> distinguishes several container categories, and the distinctions govern labelling and use.

Single-dose container

A single-unit container for an article intended for parenteral administration only, used once and discarded. Ampoules and prefilled syringes are inherently single-dose; a vial is single-dose if so labelled.

Multiple-dose container

A multiple-unit container for an article intended for parenteral administration only, permitting withdrawal of successive portions without altering the remainder.

Pharmacy bulk package

A container of a sterile preparation for parenteral use containing many single doses, intended for further dilution or dispensing in a suitable environment, and not for direct administration. Its permitted period after entry is short — commonly stated on the label and typically measured in hours.^[1]

The requirement for a preservative in a multiple-dose container appears in the general requirements for injections, and the demonstration of effectiveness is by the antimicrobial effectiveness test in USP <51>. That test challenges the formulation with specified organisms — Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Candida albicans and Aspergillus brasiliensis — and requires defined log reductions at defined intervals over 28 days. For injectable products the criteria are the most demanding of the product categories the chapter defines.^[2]

The 28-day duration of the test and the 28-day default period after puncture are related but not identical claims. The test demonstrates that the preservative system suppresses growth of a substantial deliberate inoculum over 28 days under laboratory conditions; the use period is a practice rule informed by that demonstration. Guidance is explicit that a manufacturer may specify a shorter period, and that a shorter period so specified governs.^[3][2]

Preservative-free preparations intended for parenteral use are single-dose regardless of their volume. This has a direct consequence for reconstituted peptides: a vial reconstituted with sterile water for injection contains no preservative and is, in compendial terms, a single-dose preparation, whereas one reconstituted with bacteriostatic water for injection contains benzyl alcohol and has at least the formulation characteristics of a preserved product — though not the demonstration of effectiveness, which is made for a specific formulation and not for a diluent.^[3][6]

Preservatives used in injectable products are a small set, constrained by systemic toxicity at the concentrations required, by compatibility with peptides and proteins, and by the pH range over which they remain active.

Concentrations are those conventionally used in marketed injectable products; the effective concentration is formulation-specific and must be demonstrated for the product rather than assumed from the class.^[2][7]

The phenolic preservatives illustrate that a preservative is not an inert additive. In insulin formulations, phenol and m-cresol bind at a specific site on the hexamer and shift the conformational equilibrium, and their presence is required for the physical stability of the formulation as well as for its preservation. Removing the preservative from such a product is therefore not merely a microbiological change.^[7]

Conversely, benzyl alcohol has been reported to destabilise some proteins, promoting aggregation at concentrations within the preservative range. The interaction is protein-specific, and reports concern proteins rather than short synthetic peptides, for which the equivalent question appears not to have been systematically addressed.^[8]

Preservative concentration is not constant over a container's in-use life. Volatile preservatives such as chlorobutanol are lost to the headspace and through the closure; benzyl alcohol partitions into elastomer; and adsorption onto the stopper is documented for several preservatives. Compendial expectations accordingly include a preservative content specification at the end of shelf life, not only at release, and effectiveness testing is conventionally performed at the lower end of the specified range rather than at the target concentration.^[2][9]

Repeated withdrawal has an additional effect that is easy to overlook: as liquid is removed the headspace grows, increasing both the volume into which a volatile preservative can partition and the surface area of elastomer per unit of remaining liquid. A vial near the end of its contents therefore has a somewhat less favourable preservative situation than a full one, at exactly the point at which it has accumulated the most punctures.^[9]

USP <797>, the general chapter governing sterile compounding in the United States, sets the framework within which opened containers are dated. Its treatment distinguishes conventionally manufactured products from compounded preparations, and within manufactured products distinguishes single-dose from multiple-dose containers.

Periods are those tabulated in the 2023 revision of the chapter; the chapter is periodically revised and its earlier revisions differ in detail.^[3]

Three features of this framework are consequential. The single-dose periods depend on the air quality of the environment in which the container was entered, which is a recognition that the risk being managed is airborne and personnel-borne contamination during the entry itself. The multiple-dose period does not depend on air quality, because the preservative is doing that work. And the phrase unless the manufacturer specifies otherwise is not decorative: several products carry shorter in-use periods on their labels, and those govern.^[3]

Infection-control guidance from the Centers for Disease Control and Prevention adds a practice dimension that the compendial rule does not address. Its recommendations are that a multiple-dose vial be dedicated to a single patient wherever possible; that vials be entered with a new sterile needle and syringe on every occasion; that a vial be discarded if sterility is compromised or questionable regardless of its date; and that vials entered in an immediate patient-treatment area be discarded at the end of the procedure rather than retained.^[4]

Each penetration of an elastomeric closure creates a channel that reseals by elastic recovery. The closure's ability to do so repeatedly is a specified property, tested compendially by the self-sealing test in USP <381> and the corresponding European Pharmacopoeia chapter: a defined number of penetrations is made and the container is then examined for leakage.^[10][11]

Resealing is not unconditional. Its reliability falls with the diameter of the needle used, with repeated penetration through the same point, and with damage to the elastomer surface. A closure penetrated twenty times with a 30-gauge needle at varied points is in a different condition from one penetrated twenty times with a 21-gauge needle at one point, and the compendial test — performed with a specified needle and technique — characterises the closure rather than any particular pattern of use.^[10]