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 and a needle projection aperture; said cassette unit housing cavity receiving 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, said plunger slaving part arranged such that when a drive load is applied to said rear drive-receiving face to bring the front plunger-contacting face into contact with the plunger the drive load is evenly transmitted to the plunger, wherein said circumferential wall is provided with one or more slide restrictors that restrict frictional sliding movement thereof in relation to the inner wall of the barrel. a cassette unit housing defining a cassette unit housing cavity and a needle projection aperture; said cassette unit housing cavity receiving 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, said plunger slaving part arranged such that when a drive load is applied to said rear drive-receiving face to bring the front plunger-contacting face into contact with the plunger the drive load is evenly transmitted to the plunger, wherein said circumferential wall is provided with one or more slide restrictors that restrict frictional sliding movement thereof in relation to the inner wall of the barrel. 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, said plunger slaving part arranged such that when a drive load is applied to said rear drive-receiving face to bring the front plunger-contacting face into contact with the plunger the drive load is evenly transmitted to the plunger, wherein said circumferential wall is provided with one or more slide restrictors that restrict frictional sliding movement thereof in relation to the inner wall of the barrel.