A pre-filled drug delivery device, comprising a housing, a drug container within the housing and containing a drug； a plunger held within the drug container, movement of the plunger within the drug container being operative to dispense the drug from the drug container through an outlet； and a drive mechanism within the housing including a stored energy source and a plunger rod held in locked position against the stored energy source prior to activation of the device, wherein on activation of the device the stored energy source moves the plunger rod such that the plunger rod moves the plunger within the housing wherein the plunger rod comprises a longitudinal shaft formed from sheet metal, wherein the sheet metal defines a plane, and wherein the sheet metal is deformed so that the longitudinal shaft comprises an out-of-plane engagement surface, the out-of-plane engagement surface configured to engage a retaining surface on the housing or on a component of the drive mechanism, to hold the plunger rod in the locked position.
