A syringe assembly having passive disabling structure includes a barrel (21) and a plunger rod assembly (43). The plunger rod assembly includes a plunger rod and a stopper (81) connected by an indexing locking element (71) . The number of strokes of the syringe plunger before the stopper is locked into the barrel rendering the syringe assembly unusable is determined by the number of detents (89) on the plunger rod and stopper which engage the locking mechanism. Upon completion of the final delivery stroke, any attempt to withdraw the plunger rod from the barrel will cause the locking element to engage the barrel and trap the stopper in the barrel preventing further use of the syringe.