A pressure-regulating vial adaptor comprising: a connector interface; a piercing member supported by a body portion, wherein the piercing member is in communication with the connector interface; a regulator channel extending between the piercing member and a proximal regulator aperture; and a regulator assembly comprising: a regulator base comprising an annular wall, the regulator base having a coupling protrusion configured to couple to a regulator lumen, the regulator lumen being in communication with the piercing member; a regulator nest being configured to couple with the regulator base; a flexible enclosure being positioned within a storage chamber within the regulator nest, the flexible enclosure being configured to transition between a stored configuration and an expanded configuration; and a cover being connected to the annular wall of the regulator base, the cover comprising an aperture and an annular side portion sized and shaped to fit around a radially-outward portion of the regulator base, characterized in that the annular side portion of the cover is configured to cover an outer surface of the annular wall of the regulator base, the cover being further configured to inhibit expansion of the flexible enclosure and permit the flexible enclosure to at least partially extend through the aperture as the flexible enclosure transitions from the stored configuration towards the expanded configuration; wherein the flexible enclosure is positioned within the storage chamber when the flexible enclosure is in the stored configuration and at least a portion of the flexible enclosure is positioned outside of the storage chamber when the flexible enclosure is in the expanded configuration. a connector interface; a piercing member supported by a body portion, wherein the piercing member is in communication with the connector interface; a regulator channel extending between the piercing member and a proximal regulator aperture; and a regulator assembly comprising: a regulator base comprising an annular wall, the regulator base having a coupling protrusion configured to couple to a regulator lumen, the regulator lumen being in communication with the piercing member; a regulator nest being configured to couple with the regulator base; a flexible enclosure being positioned within a storage chamber within the regulator nest, the flexible enclosure being configured to transition between a stored configuration and an expanded configuration; and a cover being connected to the annular wall of the regulator base, the cover comprising an aperture and an annular side portion sized and shaped to fit around a radially-outward portion of the regulator base, characterized in that the annular side portion of the cover is configured to cover an outer surface of the annular wall of the regulator base, the cover being further configured to inhibit expansion of the flexible enclosure and permit the flexible enclosure to at least partially extend through the aperture as the flexible enclosure transitions from the stored configuration towards the expanded configuration; wherein the flexible enclosure is positioned within the storage chamber when the flexible enclosure is in the stored configuration and at least a portion of the flexible enclosure is positioned outside of the storage chamber when the flexible enclosure is in the expanded configuration. a regulator base comprising an annular wall, the regulator base having a coupling protrusion configured to couple to a regulator lumen, the regulator lumen being in communication with the piercing member; a regulator nest being configured to couple with the regulator base; a flexible enclosure being positioned within a storage chamber within the regulator nest, the flexible enclosure being configured to transition between a stored configuration and an expanded configuration; and a cover being connected to the annular wall of the regulator base, the cover comprising an aperture and an annular side portion sized and shaped to fit around a radially-outward portion of the regulator base, characterized in that the annular side portion of the cover is configured to cover an outer surface of the annular wall of the regulator base, the cover being further configured to inhibit expansion of the flexible enclosure and permit the flexible enclosure to at least partially extend through the aperture as the flexible enclosure transitions from the stored configuration towards the expanded configuration; wherein the flexible enclosure is positioned within the storage chamber when the flexible enclosure is in the stored configuration and at least a portion of the flexible enclosure is positioned outside of the storage chamber when the flexible enclosure is in the expanded configuration.