Provided is a cassette unit suitable for use with an auto-injector having an electrically powered drive unit. The cassette unit has a housing defining a cassette unit housing cavity and a needle projection aperture. The cassette unit housing cavity is arranged for receipt of a syringe. The syringe has a barrel for containing a volume of a liquid drug formulation, the 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 the barrel, the hollow needle defining a needle tip; and a plunger that is axially movable within the barrel. The cassette unit includes, in contact with the plunger and axially movable within the barrel, a plunger slaving part arranged such that when a drive load is applied to a rear drive-receiving face thereof the drive load is evenly transmitted to the plunger.