A pen-type injection device comprising: - a housing (2, 102) extending along a general axis, - a cartridge (10, 110) comprising a cartridge body (10.1, 110.1), a penetrable septum, and a piston (15, 115), together defining a variable volume reservoir adapted to hold a liquid substance, the cartridge (10, 110) being rotationally fixed with respect to the housing (2, 102) - a dose expelling mechanism for expelling a dose of the liquid substance from the cartridge (10, 110), comprising a piston rod structure for advancing the piston (15, 115), the piston rod structure comprising ∘ a piston rod (60, 160) threadedly engaged with the housing (2, 102) and adapted to rotate and thereby undergo helical motion relative to the cartridge body (10.1, 110.1) during dose expelling, and ∘ a piston rod foot (61, 161) rotationally locked to the piston rod (60, 160) and adapted to interface with the piston (15, 115), and - output means (8.1, 8.2, 20) producing a detectable output at an angular displacement of the piston rod (60, 160) which corresponds to expelling of a predetermined dose increment, a total number of detectable outputs produced during a dose expelling event thereby being indicative of a size of an expelled dose, characterised in that the pen-type injection device further comprises a brake mechanism configured to slow down rotational motion of the piston rod (60, 160), the brake mechanism comprising a first contact surface (10.2, 110.2) forming part of the cartridge body (10.1, 110.1), and a second contact surface (15.1, 164) arranged, or adapted to be arranged, in contact with the first contact surface (10.2, 110.2) and being rotationally locked with respect to the piston rod structure. - a housing (2, 102) extending along a general axis, - a cartridge (10, 110) comprising a cartridge body (10.1, 110.1), a penetrable septum, and a piston (15, 115), together defining a variable volume reservoir adapted to hold a liquid substance, the cartridge (10, 110) being rotationally fixed with respect to the housing (2, 102) - a dose expelling mechanism for expelling a dose of the liquid substance from the cartridge (10, 110), comprising a piston rod structure for advancing the piston (15, 115), the piston rod structure comprising ∘ a piston rod (60, 160) threadedly engaged with the housing (2, 102) and adapted to rotate and thereby undergo helical motion relative to the cartridge body (10.1, 110.1) during dose expelling, and ∘ a piston rod foot (61, 161) rotationally locked to the piston rod (60, 160) and adapted to interface with the piston (15, 115), and ∘ a piston rod (60, 160) threadedly engaged with the housing (2, 102) and adapted to rotate and thereby undergo helical motion relative to the cartridge body (10.1, 110.1) during dose expelling, and ∘ a piston rod foot (61, 161) rotationally locked to the piston rod (60, 160) and adapted to interface with the piston (15, 115), and - output means (8.1, 8.2, 20) producing a detectable output at an angular displacement of the piston rod (60, 160) which corresponds to expelling of a predetermined dose increment, a total number of detectable outputs produced during a dose expelling event thereby being indicative of a size of an expelled dose, characterised in that the pen-type injection device further comprises a brake mechanism configured to slow down rotational motion of the piston rod (60, 160), the brake mechanism comprising a first contact surface (10.2, 110.2) forming part of the cartridge body (10.1, 110.1), and a second contact surface (15.1, 164) arranged, or adapted to be arranged, in contact with the first contact surface (10.2, 110.2) and being rotationally locked with respect to the piston rod structure.