A motorized drug delivery device configured to administer at least one pre-set dose of a medicament contained within a medicament container. The delivery device comprising a user activation system (13), a motor (22), a gear train (15) operatively coupled to the user activation system and the motor, and a piston plunger system (200) operatively coupled to the gear train. The piston plunger system comprises a piston plunger (50) and at least one plunger extension (42; 44) for extending a length of the piston plunger. The piston plunger system is configured to act on a stopper contained within the medicament container to deliver at least one pre-set dose of medicament. The user activation system comprises a mechanical automatic reset activation system for automatically resetting the drug delivery device after administration of the pre-set dose.