The application concerns an injection device for apportioning set doses of a drug from a reservoir (40) to a subject. The injection device comprises a housing having an interior thread (101,112) formed as an outwardly pointing thread (101,112) carried on an upstanding tower centrally located in the pen shaped device. This outwardly pointing thread (101,112) forms a first thread connection with the interior thread of the rotatable scale drum (90). The injection device further comprises a driver (70) for moving the piston rod (120) forward when moved axially. The driver (70) operates the piston rod (120) through a second thread connection having a pitch different than the first thread connection.
