A mixing device and a retractable syringe comprising same are provided. The mixing device comprises concentric outer and inner barrels that form an outer chamber, the inner barrel having an inner chamber. A mixing plunger is slidably located in the outer chamber. A seal located in the outer chamber is capable of axial movement, in response to depression of the mixing plunger, from a first position in sealing engagement with one or more apertures in the inner barrel to a second position intermediate the apertures and vents in the outer barrel. This allows depression of the mixing plunger to force a first substance from the outer chamber through the apertures to mix with a second substance in the inner chamber. The mixed substance in the inner barrel is then delivered by the syringe with subsequent needle retraction.