Abstract: A dual chamber syringe facilitates selective drug mixing, dosing, and administration through catheters or other vascular accessing devices (VADs), as well as pre- or post-administration flushing of catheters with a single syringe instrument. A syringe barrel incorporates an outlet, such as a Luer connector, variable-volume, axially tandem primary and secondary fluid chambers, with nested respective plungers, plunger rods capable of selective translation within the barrel interior along a common axis, and plunger stoppers. The secondary fluid chamber is incorporated within the primary plunger. A selectively resealable valve disposed within the barrel interior, selectively isolates the secondary fluid chamber from fluid communication with the primary fluid chamber and/or the outlet of the syringe barrel. In some embodiments, the valve is a duckbill-type valve or an umbrella valve. In some embodiments, the syringe is prepackaged with flushing solution in the secondary fluid chamber.