Retractable syringe with locking plunger

A syringe, comprising:

• a barrel having a proximal end and a distal end, and an interior surface defining a cavity configured to hold a medicant;
• a plunger having an annular seal disposed in the barrel and configured to advance toward the barrel distal end;
• a needle stem disposed in the barrel distal end;
• a needle coupled to the needle stem and configured to deliver the medicant upon advancement of the plunger toward the barrel distal end, the needle stem and needle configured to retract into the barrel after delivery of the medicant;
• and
• wherein the barrel interior surface comprises a retaining ledge at the barrel distal end, wherein the annular seal is configured to advance distal of the retaining ledge after retraction of the needle and needle stem, and wherein the retaining ledge is configured to secure the plunger in the barrel distal end, wherein the retaining ledge comprises a plurality of members extending inwardly from the barrel interior surface with a space defined between each of the members.