A single-use auto-disable syringe device of reduced size utilizes a locking element to prevent re-use of the syringe by locking the plunger rod with respect to the barrel upon completion of an injection stroke. The plunger rod includes a plurality of teeth comprising distally facing shoulders provided on a majority of the length of an elongate portion of the plunger rod and a breakable section configured to break upon the application of excessive axial or torsional force. The breakable section is preferably provided at a distal end of the plunger rod so that a proximal end of the locking element engages the syringe barrel and plunger rod at a position more proximal than the breakable section. In this way, the locking element resists some of the force that would otherwise be applied to the breakable section if the plunger rod were twisted. The plunger rod also includes an intermediate tooth provided on a distal-most tooth to prevent the user from repeatedly drawing in doses too small to activate the safety mechanism.
