An auto-injector for hypodermic delivery of medication includes a first subassembly releasably coupled to a second subassembly. The first subassembly includes a cartridge holder configured to receive a medication cartridge, an injection needle through which medication can pass from the medication cartridge, and a needle shield movable between an extended position enclosing the needle and a retracted position in which a proximal end of the needle is not enclosed by the needle shield. The second subassembly includes a movable plunger rod positioned within a housing. Movement of the needle shield in a proximal direction from the retracted position to the extended position following administration of the medication automatically releases the first subassembly from the second subassembly. The released first subassembly is locked with the needle shield in the extended position. A new first subassembly containing an unused dosage of medication may be inserted and releasably connected to the second subassembly.
