A hypodermic syringe has an elongated tubular body having a front end adapted to carry a needle, a rear end, and a bypass between the ends. A front partition piston defines with the front end a front compartment adapted to hold a substance and a rear piston defines with the front piston a rear compartment adapted to hold a fluid miscible with the front-compartment substance. The front piston is displaceable into a middle position in the bypass for fluid communication between the compartments. A stem projecting rearward out of the body from the rear piston is provided with axially spaced front and rear external screwthreads and has a clear region between the screwthreads. A damper on the rear end of the body can engage the screwthreads for slowing axial movement of the screwthreads past the rear end of the body and for permitting relatively rapid axial movement of the stem in the body when the clear region is level with the damper.