A syringe assembly comprising: a barrel including a cylindrical sidewall having an interior surface 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; an elongate plunger rod including a proximal end, a distal end, and a main body extending between the proximal and distal end, 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 protrusion between the thumb press and the main body, the protrusion having a diameter greater than the diameter of the barrel at the rib and the plunger rod further including at least one frangible portion; a stopper having a proximal end and a distal end, the stopper attached to the stopper-engaging portion of the plunger rod, the stopper being distally and proximally movable relative to the stopper-engaging portion for a pre-selected axial distance such that when the distal end of the stopper is in contact with the distal wall of the barrel, the 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, characterized in that, said sidewall including a diameter transition region having an axial length extending from the rib towards said proximal end such that the diameter of the barrel increases along the transition region from the rib towards the proximal end, said stopper having a peripheral edge that forms a seal with the interior surface of the barrel having a diameter greater than the diameter of the barrel at the rib and the stopper-engaging portion comprising a visual indicator that is fully visible before an initial distally directed force is applied to the plunger rod. a barrel including a cylindrical sidewall having an interior surface 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; an elongate plunger rod including a proximal end, a distal end, and a main body extending between the proximal and distal end, 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 protrusion between the thumb press and the main body, the protrusion having a diameter greater than the diameter of the barrel at the rib and the plunger rod further including at least one frangible portion; a stopper having a proximal end and a distal end, the stopper attached to the stopper-engaging portion of the plunger rod, the stopper being distally and proximally movable relative to the stopper-engaging portion for a pre-selected axial distance such that when the distal end of the stopper is in contact with the distal wall of the barrel, the 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, characterized in that, said sidewall including a diameter transition region having an axial length extending from the rib towards said proximal end such that the diameter of the barrel increases along the transition region from the rib towards the proximal end, said stopper having a peripheral edge that forms a seal with the interior surface of the barrel having a diameter greater than the diameter of the barrel at the rib and the stopper-engaging portion comprising a visual indicator that is fully visible before an initial distally directed force is applied to the plunger rod.