A cassette unit for use with an auto-injector having an electrically powered drive unit, said cassette unit comprising: a cassette unit housing defining a cassette unit housing cavity, a rearward entrance to said cavity and a forward needle projection aperture; said cassette unit housing cavity arranged for receipt of a syringe comprising: a barrel for containing a volume of a liquid drug formulation, said barrel defining a flange at the rear end thereof and a forward shoulder at the forward end thereof; a hollow needle at a front end of said barrel, said hollow needle defining a needle tip for dispensing of said liquid drug formulation; and a plunger that is axially movable within the barrel, characterized by comprising: axially movable within the barrel for forward movement into contact with said plunger, a plunger slaving part defining a circumferential wall arranged for frictional sliding relationship with the inner wall of the barrel, a rear drive-receiving face and a front plunger-contacting face; in capping relationship with said rearward entrance of the cassette unit housing, a cassette unit end-cap, said cassette unit end-cap defining a drive rod-receiving opening for receipt of a drive rod for providing forward axial drive to said plunger slaving part, wherein in a pre-use configuration, the plunger slaving part is shaped for releasable engagement with the cassette unit end-cap. a cassette unit housing defining a cassette unit housing cavity, a rearward entrance to said cavity and a forward needle projection aperture; said cassette unit housing cavity arranged for receipt of a syringe comprising: a barrel for containing a volume of a liquid drug formulation, said barrel defining a flange at the rear end thereof and a forward shoulder at the forward end thereof; a hollow needle at a front end of said barrel, said hollow needle defining a needle tip for dispensing of said liquid drug formulation; and a plunger that is axially movable within the barrel, a barrel for containing a volume of a liquid drug formulation, said barrel defining a flange at the rear end thereof and a forward shoulder at the forward end thereof; a hollow needle at a front end of said barrel, said hollow needle defining a needle tip for dispensing of said liquid drug formulation; and a plunger that is axially movable within the barrel, characterized by comprising: axially movable within the barrel for forward movement into contact with said plunger, a plunger slaving part defining a circumferential wall arranged for frictional sliding relationship with the inner wall of the barrel, a rear drive-receiving face and a front plunger-contacting face; in capping relationship with said rearward entrance of the cassette unit housing, a cassette unit end-cap, said cassette unit end-cap defining a drive rod-receiving opening for receipt of a drive rod for providing forward axial drive to said plunger slaving part, wherein in a pre-use configuration, the plunger slaving part is shaped for releasable engagement with the cassette unit end-cap. axially movable within the barrel for forward movement into contact with said plunger, a plunger slaving part defining a circumferential wall arranged for frictional sliding relationship with the inner wall of the barrel, a rear drive-receiving face and a front plunger-contacting face; in capping relationship with said rearward entrance of the cassette unit housing, a cassette unit end-cap, said cassette unit end-cap defining a drive rod-receiving opening for receipt of a drive rod for providing forward axial drive to said plunger slaving part, wherein in a pre-use configuration, the plunger slaving part is shaped for releasable engagement with the cassette unit end-cap.