Provided is a cassette unit suitable for use with an auto-injector having an electrically powered drive unit. The cassette unit comprises a cassette unit housing defining a cassette unit housing cavity and a needle projection aperture. The housing cavity may receive a syringe. Axially movable within the barrel of the syringe for forward movement into contact with the syringe plunger, there is 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. The plunger slaving part is arranged such that when a drive load is applied to its rear drive-receiving face to bring the front plunger-contacting face into contact with the plunger the drive load is evenly transmitted. The circumferential wall is provided with slide restrictors that restrict frictional sliding movement thereof in relation to the inner wall of the barrel.