An autoinjector for expelling a dose of drug from a held cartridge, the autoinjector comprising: - a base (200, 220, 1200) forming a housing, - a needle that is fixedly mounted relative to the base (200, 220, 1200) and comprising a front needle; - a drug cartridge arranged relative to the base (200, 220, 1200), the cartridge comprising: a) an elongated body having a distal end and a proximal end and defining a central longitudinal axis, the body having a distally arranged outlet adapted for connection to the needle, and b) a piston accommodated in the body, the piston configured for being driven axially in the distal direction to expel a dose of a drug through the outlet, - a plunger (310, 320, 400, 1320) adapted for cooperation with the piston, - an actuating spring provided as a helical compression spring arranged to act on the plunger (310, 320, 400, 1320) by exerting an axial force on the plunger (310, 320, 400, 1320) to drive the piston distally, - a needle shield (350, 380, 1380) axially movable relative to the base (200, 220, 1200) between an extended position and a collapsed position, wherein the autoinjector defines a lock (320, 328, 380, 388, 1380, 1388, 1204, 1208) configured for releasably maintaining the plunger (310, 320, 400, 1320) in an initial axial position where the actuating spring is tensioned, the lock (320, 328, 380, 388, 1380, 1388, 1204, 1208) being operated by the needle shield (350, 380, 1380), wherein manual operation of the needle shield (350, 380, 1380) by movement of the needle shield (350, 380, 1380) relative to the front needle towards the collapsed position causes manual penetration of the front needle into the injection site and causes subsequent release of the lock (320, 328, 380, 388, 1380, 1388, 1204, 1208), wherein the plunger (310, 320, 400, 1320) defines a plunger thread (325, 1325) and the base (200, 220, 1200) defines a base thread (205, 1205) adapted to cooperate with the plunger thread (325, 1325), wherein prior to activation, a) the plunger thread (325, 1325) engages the base thread (205, 1205) and b) the lock (320, 328, 380, 388, 1380, 1388, 1204, 1208) acts to prevent relative rotation between the plunger (310, 320, 400, 1320) and the base (200, 220, 1200), thereby maintaining the plunger (310, 320, 400, 1320) in the initial axial position, and wherein, upon the needle shield (350, 380, 1380) being moved towards the collapsed position, the lock (320, 328, 380, 388, 1380, 1388, 1204, 1208) is released to enable the force exerted by the actuating spring to cause relative rotation between the plunger (310, 320, 400, 1320) and the base (200, 220, 1200) causing release of the plunger (310, 320, 400, 1320) from the initial axial position and expelling the dose of the drug, characterized in that the autoinjector is for expelling a single dose of drug from the held cartridge, wherein the plunger thread (325, 1325) is engaged with the base thread during an initial first axial displacement of the plunger (310, 320, 400, 1320) and wherein the plunger thread (325, 1325) is released from engagement with the base thread (205, 1205) allowing the plunger (310, 320, 400) to subsequently continue axial displacement in a second axial displacement. - a base (200, 220, 1200) forming a housing, - a needle that is fixedly mounted relative to the base (200, 220, 1200) and comprising a front needle; - a drug cartridge arranged relative to the base (200, 220, 1200), the cartridge comprising: a) an elongated body having a distal end and a proximal end and defining a central longitudinal axis, the body having a distally arranged outlet adapted for connection to the needle, and b) a piston accommodated in the body, the piston configured for being driven axially in the distal direction to expel a dose of a drug through the outlet, a) an elongated body having a distal end and a proximal end and defining a central longitudinal axis, the body having a distally arranged outlet adapted for connection to the needle, and b) a piston accommodated in the body, the piston configured for being driven axially in the distal direction to expel a dose of a drug through the outlet, - a plunger (310, 320, 400, 1320) adapted for cooperation with the piston, - an actuating spring provided as a helical compression spring arranged to act on the plunger (310, 320, 400, 1320) by exerting an axial force on the plunger (310, 320, 400, 1320) to drive the piston distally, - a needle shield (350, 380, 1380) axially movable relative to the base (200, 220, 1200) between an extended position and a collapsed position,