Disclosed is a syringe assembly which includes a barrel with a cylindrical sidewall having an interior surface defining a chamber for retaining fluid, an open proximal end and a distal end. The distal end has a distal wall with an opening therethrough and in fluid communication with the chamber. The assembly includes an elongate plunger rod with a proximal end, a distal end and a main body extending between the proximal and distal end. The plunger rod is distally and proximally movable within the chamber. The proximal end has a thumb press with a proximal end, a distal end and a diameter. The distal end of the plunger rod has a stopper-engaging portion. The assembly has an annular extension protruding proximally from the proximal end of the barrel. The annular extension has a diameter greater than the diameter of the thumb press. The assembly further includes a stopper with a proximal end and a distal end. The stopper is attached to the stopper engaging portion of the plunger rod and is distally and proximally movable relative to the stopper engaging portion for a pre-selected axial distance. When the distal end of the stopper is in contact with the distal wall of the barrel, the thumb press is permitted to advance distally into the annular extension. The annular extension partially envelopes a portion of the thumb press to prevent the user from accessing the thumb press and moving the plunger rod in a proximal direction. The stopper engaging portion has a visual indicator which is fully visible when the stopper engaging portion is proximally moved relative to the stopper.
