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 pen shaped device. This outwardly pointing thread forms a first thread connection with the interior thread of the rotatable scale drum. The injection device further comprises a driver for moving the 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.