Syringe assemblies having a passive disabling system to prevent reuse are provided. According to one or more embodiments, the syringe assembly comprises a barrel, plunger rod and detachable thumb press wherein the plunger rod further comprises flexible locking petals that lock the plunger rod within the barrel and one or more frangible connection or break point at the proximal end of the plunger rod disposed either inside the thumb press or immediately adjacent to the thumb press to separate the thumb press from the main body of the plunger rod when a user applies sufficient proximal force to the plunger rod after it has been locked within the barrel.
