The invention concerns an injection device for apportioning set doses of a drug from a reservoir to a subject. The injection device comprises a housing having an interior thread formed as an outwardly pointing thread carried on an upstanding tower centrally located in the injection device. The outwardly pointing thread forms a first thread connection with an interior thread of a rotatable scale drum. The injection device further comprises a driver for moving a piston rod forward when moved axially. The driver operates the piston rod through a second thread connection having a pitch different than the first thread connection.