A medical adaptor capable of withdrawing medical substance from a vial, the medical adaptor comprising: a medical connector interface; an access channel through which medicinal fluid from a sealed container can be removed, the access channel extending between the medical connector interface and a distal access port; a regulator channel comprising a distal portion and a proximal portion; and a regulator assembly in fluid communication with the proximal portion of the regulator channel, the regulator assembly comprising: a storage region having a storage width and an internal storage volume: a flexible enclosure in fluid communication with the proximal portion of the regulator channel, the flexible enclosure capable of transitioning between an initial contracted configuration in which the flexible enclosure is positioned within the storage region and is in a folded contracted configuration and a deployed configuration in which the flexible enclosure is expanded, the flexible enclosure having a deployed volume and a deployed width in the deployed configuration; and an enclosure cover fitted around the storage region and configured to inhibit or prevent the flexible enclosure from rapidly transitioning to the deployed configuration; an intake valve in fluid communication with the flexible enclosure and capable of fluid communication with the regulator channel, the intake valve configured to transition from a closed configuration to an open configuration when fluid is withdrawn from the vial while the flexible enclosure is in the stored configuration; wherein: the flexible enclosure is configured to permit withdrawal of a first volume of medical fluid from the vial and configured to regulate pressure within the vial while the flexible enclosure is in the deployed configuration and the intake valve is in the closed configuration; and the intake valve is configured to permit withdrawal of fluid from the vial and configured to regulate pressure within the vial while the flexible enclosure is in the initial contracted configuration and the valve permits passage of fluid. a medical connector interface; an access channel through which medicinal fluid from a sealed container can be removed, the access channel extending between the medical connector interface and a distal access port; a regulator channel comprising a distal portion and a proximal portion; and a regulator assembly in fluid communication with the proximal portion of the regulator channel, the regulator assembly comprising: a storage region having a storage width and an internal storage volume: a flexible enclosure in fluid communication with the proximal portion of the regulator channel, the flexible enclosure capable of transitioning between an initial contracted configuration in which the flexible enclosure is positioned within the storage region and is in a folded contracted configuration and a deployed configuration in which the flexible enclosure is expanded, the flexible enclosure having a deployed volume and a deployed width in the deployed configuration; and an enclosure cover fitted around the storage region and configured to inhibit or prevent the flexible enclosure from rapidly transitioning to the deployed configuration; an intake valve in fluid communication with the flexible enclosure and capable of fluid communication with the regulator channel, the intake valve configured to transition from a closed configuration to an open configuration when fluid is withdrawn from the vial while the flexible enclosure is in the stored configuration; a storage region having a storage width and an internal storage volume: a flexible enclosure in fluid communication with the proximal portion of the regulator channel, the flexible enclosure capable of transitioning between an initial contracted configuration in which the flexible enclosure is positioned within the storage region and is in a folded contracted configuration and a deployed configuration in which the flexible enclosure is expanded, the flexible enclosure having a deployed volume and a deployed width in the deployed configuration; and an enclosure cover fitted around the storage region and configured to inhibit or prevent the flexible enclosure from rapidly transitioning to the deployed configuration; an intake valve in fluid communication with the flexible enclosure and capable of fluid communication with the regulator channel, the intake valve configured to transition from a closed configuration to an open configuration when fluid is withdrawn from the vial while the flexible enclosure is in the stored configuration; wherein: the flexible enclosure is configured to permit withdrawal of a first volume of medical fluid from the vial and configured to regulate pressure within the vial while the flexible enclosure is in the deployed configuration and the intake valve is in the closed configuration; and the intake valve is configured to permit withdrawal of fluid from the vial and configured to regulate pressure within the vial while the flexible enclosure is in the initial contracted configuration and the valve permits passage of fluid. the flexible enclosure is configured to permit withdrawal of a first volume of medical fluid from the vial and configured to regulate pressure within the vial while the flexible enclosure is in the deployed configuration and the intake valve is in the closed configuration; and the intake valve is configured to permit withdrawal of fluid from the vial and configured to regulate pressure within the vial while the flexible enclosure is in the initial contracted configuration and the valve permits passage of fluid.