A syringe assembly comprising: a cylinder (120) that includes a cylindrical side wall (110) having an inner surface (126) defining a chamber (128) for retaining fluid, an open proximal end (129) and an end distal (121) including a distal wall (122) having an opening therethrough in fluid communication with said chamber (128), said side wall (110) including a rib (123) adjacent to said proximal end (129) ； an elongated piston rod (140) including a proximal end (149), a distal end (141) and a main body (148) extending between the proximal and distal end, the piston rod (140) being distal and proximally movable within said chamber (128), including the proximal end (149) a thumb press (142) including the distal end (141) a plug coupling part (146), including the piston rod (140) in addition a protrusion (144) between the thumb press (142) and the main body (148), the protrusion (144) having a diameter larger than the diameter of the cylinder (120) in the rib (123) and including the stem of the plunger (140) in addition to at least one frangible part (143)； a plug (160) having a proximal end (169) and a distal end (161), the plug (160) attached to the coupling part (146) of the piston rod plug (140), the plug (146) being ) distal and proximally movable relative to the coupling portion (146) of the plug for a preselected axial distance so that when the distal end (161) of the plug (160) is in contact with the distal wall (122) of the cylinder ( 120), the protrusion (144) is allowed to advance distally beyond the rib (123) in the cylinder (120) and block the piston rod (140) in the cylinder (120) to prevent reuse of the syringe assembly , characterized in that, said side wall (110) includes a transition region of diameter (127) having an axial length extending from the rib (123) towards said proximal end (129) so that the diameter of the cylinder ( 120) increases along the transition region (127) from the nerve adura (123) towards the proximal end (129), said plug (160) ha
