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 having a proximally facing surface having a surface area disposed proximally adjacent to the flexible protrusion and at least one frangible portion disposed proximally adjacent to the support member, the frangible portion comprising two or more point connections connecting the support member to the proximal end of the plunger rod, each point connection including a body portion extending from a distal end of the point connection to a proximal end of the point connection which decreases in cross-sectional width from the distal end of the point connection to the proximal end of the point connection, the distal ends of the two or more point connections collectively defining a point connection surface having a surface area that is attached to the proximally facing surface of the support member such that the ratio of the surface area of the proximally facing surface to the surface area of the point connection surface is at least 200; 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; and a gap between the distal end of the stopper and the distal wall of the barrel when the syringe assembly is in an initial position. 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 having a proximally facing surface having a surface area disposed proximally adjacent to the flexible protrusion and at least one frangible portion disposed proximally adjacent to the support member, the frangible portion comprising two or more point connections connecting the support member to the proximal end of the plunger rod, each point connection including a body portion extending from a distal end of the point connection to a proximal end of the point connection which decreases in cross-sectional width from the distal end of the point connection to the proximal end of the point connection, the distal ends of the two or more point connections collectively defining a point connection surface having a surface area that is attached to the proximally facing surface of the support member such that the ratio of the surface area of the proximally facing surface to the surface area of the point connection surface is at least 200; 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; and a gap between the distal end of the stopper and the distal wall of the barrel when the syringe assembly is in an initial position.