In certain embodiments, a vial adaptor comprises a housing configured to couple the adaptor with a vial, an access channel, a regulator channel, and a regulator assembly. The access channel is configured to facilitate withdrawal of fluid from the vial when the adaptor is coupled to the vial. The regulator channel is configured to facilitate a flow of a regulating fluid from the regulator assembly to compensate for changes in volume of a medical fluid in the vial. In some embodiments, the regulator assembly includes a flexible member configured to expand and contract in accordance with changes in the volume of the medical fluid in the vial. In some embodiments, the flexible member is substantially free to expand and contract. In some embodiments, the flexible member is not partly or completely located in a rigid enclosure.