A syringe assembly comprising: a barrel including a cylindrical sidewall having an interior surface with a first cross-sectional width defining a chamber for retaining fluid, an open proximal end and a distal end including a distal wall having an opening therethrough in fluid communication with said chamber, said sidewall including a rib adjacent said proximal end defining a second cross-sectional width that is less than the first cross-sectional width; an elongate plunger rod including a proximal end, a distal end, and a main body extending between the proximal and distal end (549, 541), the plunger rod being distally and proximally movable within said chamber, the proximal end including a thumb press, the distal end including a stopper-engaging portion, the plunger rod further including a flexible protrusion between the thumb press and the main body, the protrusion having a cross-sectional width greater than the cross-sectional width of the barrel at the rib and the plunger rod further including a support member including an outer edge disposed proximally adjacent to the flexible protrusion and at least one frangible portion disposed proximally adjacent to the support member and comprising at least four point connections (572, 573, 574, 575) with a first point connection, a second point connection, a third point connection, and a fourth point connection disposed adjacent to the edge of the support member; the protrusion including four leaves (551, 552, 553, 554) extending radially outwardly from the plunger rod and being disposed equidistant from each other and wherein the first point connection and second point connection are aligned with a first and second leaves (551, 553), the third point connection and the fourth point connection are misaligned with a third and a fourth leaves (552, 554), wherein the third point connection and the fourth point connection are disposed adjacent to opposite ends of the outer edge of the support member; a stopper having a proximal end and a distal end, the stopper attached to the stopper-engaging portion of the plunger rod such that when the distal end of the stopper is in contact with the distal wall of the barrel, the flexible protrusion is permitted to advance distally past the rib in the barrel and lock the plunger rod in the barrel to prevent reuse of the syringe assembly, wherein application of a proximally-direceted force to the plunger, after the protrusion has advanced distally past the rib, causes the frangible portion of the plunger rod to break, characterised in that the protrusion curves outwardly from plunger rod toward the proximal end of the plunger rod such that each of the leaves has an arcuate shape, so that the flexible protrusion is inhibited from flexing inwardly after the plunger rod has been locked. a barrel including a cylindrical sidewall having an interior surface with a first cross-sectional width defining a chamber for retaining fluid, an open proximal end and a distal end including a distal wall having an opening therethrough in fluid communication with said chamber, said sidewall including a rib adjacent said proximal end defining a second cross-sectional width that is less than the first cross-sectional width; an elongate plunger rod including a proximal end, a distal end, and a main body extending between the proximal and distal end (549, 541), the plunger rod being distally and proximally movable within said chamber, the proximal end including a thumb press, the distal end including a stopper-engaging portion, the plunger rod further including a flexible protrusion between the thumb press and the main body, the protrusion having a cross-sectional width greater than the cross-sectional width of the barrel at the rib and the plunger rod further including a support member including an outer edge disposed proximally adjacent to the flexible protrusion and at least one frangible portion disposed proximally adjacent to the support member and comprising at least four point connections (572, 573, 574, 575) with a first point connection, a second point connection, a third point connection, and a fourth point connection disposed adjacent to the edge of the support member; the protrusion including four leaves (551, 552, 553, 554) extending radially outwardly from the plunger rod and being disposed equidistant from each other and wherein the first point connection and second point connection are aligned with a first and second leaves (551, 553), the third point connection and the fourth point connection are misaligned with a third and a fourth leaves (552, 554), wherein the third point connection and the fourth point connection are disposed adjacent to opposite ends of the outer edge of the support member; a stopper having a proximal end and a distal end, the stopper attached to the stopper-engaging portion of the plunger rod such that when the distal end of the stopper is in contact with the distal wall of the barrel, the flexible protrusion is permitted to advance distally past the rib in the barrel and lock the plunger rod in the barrel to prevent reuse of the syringe assembly, wherein application of a proximally-direceted force to the plunger, after the protrusion has advanced distally past the rib, causes the frangible portion of the plunger rod to break, characterised in that the protrusion curves outwardly from plunger rod toward the proximal end of the plunger rod such that each of the leaves has an arcuate shape, so that the flexible protrusion is inhibited from flexing inwardly after the plunger rod has been locked.