A module for use in a combinatorial drug delivery device, the module formed to accommodate a drug vial with a septum, the module comprising: a vial spike formed to pierce the septum of the drug vial with a free end of the vial spike configured to be located interiorly of the septum, the vial spike including an inlet path open at the free end and extending along the vial spike to an inlet opening, and an outlet path open at the free end and extending along the vial spike to an outlet chamber, the outlet path being separate from the inlet path; a sealing port selectively sealing the inlet opening; characterized in that the module further comprises: a vent in communication with the inlet path between the inlet opening and the free end; a slidable piston valve located in the outlet chamber, the piston valve forming a seal in the outlet chamber to define first and second chamber portions (6a, 6b), the outlet path being in communication with the first chamber portion; an outlet fitting in communication with the second chamber portion; and, a bypass passageway extending between the outlet fitting and an opening in the outlet chamber, wherein, in an initial state, the piston valve is located in a first position where the first chamber portion is sealed from the bypass passageway, and, wherein, with negative pressure introduced through the outlet fitting, the piston valve is caused to move to a second position where the first chamber portion is in communication with the bypass passageway. a vial spike formed to pierce the septum of the drug vial with a free end of the vial spike configured to be located interiorly of the septum, the vial spike including an inlet path open at the free end and extending along the vial spike to an inlet opening, and an outlet path open at the free end and extending along the vial spike to an outlet chamber, the outlet path being separate from the inlet path; a sealing port selectively sealing the inlet opening; characterized in that the module further comprises: a vent in communication with the inlet path between the inlet opening and the free end; a slidable piston valve located in the outlet chamber, the piston valve forming a seal in the outlet chamber to define first and second chamber portions (6a, 6b), the outlet path being in communication with the first chamber portion; an outlet fitting in communication with the second chamber portion; and, a bypass passageway extending between the outlet fitting and an opening in the outlet chamber, wherein, in an initial state, the piston valve is located in a first position where the first chamber portion is sealed from the bypass passageway, and, wherein, with negative pressure introduced through the outlet fitting, the piston valve is caused to move to a second position where the first chamber portion is in communication with the bypass passageway. a vent in communication with the inlet path between the inlet opening and the free end; a slidable piston valve located in the outlet chamber, the piston valve forming a seal in the outlet chamber to define first and second chamber portions (6a, 6b), the outlet path being in communication with the first chamber portion; an outlet fitting in communication with the second chamber portion; and, a bypass passageway extending between the outlet fitting and an opening in the outlet chamber, wherein, in an initial state, the piston valve is located in a first position where the first chamber portion is sealed from the bypass passageway, and, wherein, with negative pressure introduced through the outlet fitting, the piston valve is caused to move to a second position where the first chamber portion is in communication with the bypass passageway.