A dual vial adapter assemblage for use with a drug vial, a liquid vial and a needleless syringe, the drug vial having a drug vial bottle and a drug vial stopper sealing the drug vial bottle, the drug vial containing a medicament, the liquid vial having a liquid vial bottle and a liquid vial stopper sealing the liquid vial bottle, the liquid vial containing liquid contents for mixing with or reconstituting the medicament in the drug vial to form a liquid drug therein, the needleless syringe being empty, the dual vial adapter assemblage having a longitudinal dual vial adapter assemblage centerline and comprising: (a) a vented drug vial adapter having a longitudinal drug vial adapter centerline and including a transverse drug vial adapter top wall with an upright drug vial adapter connector, an oppositely directed drug vial adapter skirt for telescopic mounting on the drug vial, and a dual lumen drug vial stopper puncturing cannula for puncturing the drug vial stopper on said telescopic mounting said vented drug vial adapter on the drug vial, said dual lumen drug vial stopper puncturing cannula having i) a liquid lumen in flow communication with said drug vial adapter connector and ii) a vented air lumen openly vented to the ambient surroundings; and (b) a vented liquid vial adapter having a longitudinal liquid vial adapter centerline and including a transverse liquid vial adapter top wall with an upright liquid vial adapter connector, an oppositely directed liquid vial adapter skirt for telescopic mounting on the liquid vial, and a dual lumen liquid vial stopper puncturing cannula for puncturing the liquid vial stopper on said telescopic mounting said vented liquid vial adapter on the liquid vial, said dual lumen liquid vial stopper puncturing cannula having i) a liquid lumen in flow communication with said liquid vial adapter connector and ii) a vented air lumen openly vented to the ambient surroundings, the vented drug vial adapter and the vented liquid vial adapter being formed as discrete components, the arrangement being such that in a set-up position of the dual vial adapter assemblage in which said drug vial adapter connector is connected to, and in flow communication with, said liquid vial adapter connector, on initial telescopic mounting said vented liquid vial adapter on the liquid vial and said vented drug vial adapter on the drug vial, subsequent disposing of the liquid vial above the drug vial leads to gravitational flow of liquid contents from the liquid vial into the drug vial for forming the liquid drug therein assisted by simultaneous venting of the liquid vial by drawing ambient air thereinto through said dual lumen liquid vial stopper puncturing cannula's vented air lumen and the drug vial by expelling air therefrom through said dual lumen drug vial stopper puncturing cannula's vented air lumen to the ambient surroundings, the drug vial adapter being detachable from the liquid vial adapter for providing access to the drug vial adapter connector for enabling connection of the empty needleless syringe thereto for aspirating a liquid drug dosage from the drug vial for administration to a patient. (a) a vented drug vial adapter having a longitudinal drug vial adapter centerline and including a transverse drug vial adapter top wall with an upright drug vial adapter connector, an oppositely directed drug vial adapter skirt for telescopic mounting on the drug vial, and a dual lumen drug vial stopper puncturing cannula for puncturing the drug vial stopper on said telescopic mounting said vented drug vial adapter on the drug vial, said dual lumen drug vial stopper puncturing cannula having i) a liquid lumen in flow communication with said drug vial adapter connector and ii) a vented air lumen openly vented to the ambient surroundings; and (b) a vented liquid vial adapter having a longitudinal liquid vial adapter centerline and including a transverse liquid vial adapter top wall with an upright liquid vial adapter connector, an oppositely directed liquid vial adapter skirt for telescopic mounting on the liquid vial, and a dual lumen liquid vial stopper puncturing cannula for puncturing the liquid vial stopper on said telescopic mounting said vented liquid vial adapter on the liquid vial, said dual lumen liquid vial stopper puncturing cannula having i) a liquid lumen in flow communication with said liquid vial adapter connector and ii) a vented air lumen openly vented to the ambient surroundings,