An autoinjector (100) for expelling a single dose of drug from a drug cartridge (600) including a piston (630), the autoinjector including: a base (200, 220), a needle (500) that is fixedly mounted relative to the base (200, 220), a plunger (310, 320, 400) adapted for cooperation with the piston (630), an actuating spring (330) provided as a helical compression spring arranged to act on the plunger (310, 320, 400) by exerting an axial force on the plunger to drive the piston (630) distally, and a needle shield (350, 380) axially movable relative to the base (200, 220) between an extended position and a collapsed position. The autoinjector defines a lock (320, 328, 380, 388) that is released when the needle shield (350, 380) is moved from the extended position to the collapsed position. The lock (320, 328, 380, 388) releases to enable relative rotation between plunger thread (325) and base thread (205) causing release of the plunger (310, 320, 400) from the initial axial position and expelling the dose of the drug.
