The present invention relates to a handheld injection device comprising a housing (10), a piston rod (130) located within the housing (10), a drive member (100) and a power reservoir (110) for driving the drive member (100). The drive member (100) is permanently coupled to the piston rod (130), with the drive member (100) being axially movable between a dose setting position, in which the drive member (100) is rotationally constrained to the housing (10), and a dose dispensing position, in which the drive member (100) is rotationally decoupled from the housing (10). The power reservoir (110) comprises a reverse wound flat spiral spring having a first end attached to a first spool (120) and a second end attached to a second spool, which is axially and rotationally constrained to drive member (100).
