An autoinjector for expelling a dose of drug from a held cartridge , the autoinjector comprising: - a base forming a housing, - a needle that is fixedly mounted relative to the base and comprising a front needle ; - a drug cartridge arranged relative to the base , 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 adapted for cooperation with the piston , - an actuating spring provided as a helical compression spring arranged to act on the plunger by exerting an axial force on the plunger to drive the piston distally, - a needle shield axially movable relative to the base between an extended position and a collapsed position, wherein the autoinjector defines a lock configured for releasably maintaining the plunger in an initial axial position where the actuating spring is tensioned, the lock being operated by the needle shield , wherein manual operation of the needle shield by movement of the needle shield 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 , wherein the plunger defines a plunger thread and the base defines a base thread adapted to cooperate with the plunger thread , wherein prior to activation, a) the plunger thread engages the base thread and b) the lock acts to prevent relative rotation between the plunger and the base , thereby maintaining the plunger in the initial axial position, and wherein, upon the needle shield being moved towards the collapsed position, the lock is released to enable the force exerted by the actuating spring to cause relative rotation between the plunger and the base causing release of the plunger 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 is engaged with the base thread during an initial first axial displacement of the plunger and wherein the plunger thread is released from engagement with the base thread allowing the plunger to subsequently continue axial displacement in a second axial displacement. - a base forming a housing, - a needle that is fixedly mounted relative to the base and comprising a front needle ; - a drug cartridge arranged relative to the base , 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 adapted for cooperation with the piston , - an actuating spring provided as a helical compression spring arranged to act on the plunger by exerting an axial force on the plunger to drive the piston distally, - a needle shield axially movable relative to the base between an extended position and a collapsed position, <EOS>