Medicament admixing system

A stopper for fluidly sealing a pharmaceutical vial having a body portion defining an internal cavity, a neck portion defining an opening in fluid communication with the internal cavity, and an upper surface adjacent said opening, said stopper comprising:

• a body portion constructed to be inserted into a neck portion of a pharmaceutical vial through an opening of a pharmaceutical vial, said body portion constructed to frictionally engage a pharmaceutical vial adjacent to an opening of a pharmaceutical vial and to fluidly seal said pharmaceutical vial, said body portion having a first end portion;
• a flange portion extending radially outwardly from said first end portion of said body portion of said stopper, said flange having an upper surface and a lower surface, said lower surface constructed to engage an upper surface of a pharmaceutical vial, an undercut provided at an intersection of said lower surface of said flange portion and said body portion, said undercut constructed to enable said flange to move upwardly relative to said body portion from a first position in which said flange portion extends from said body portion at an angle that is substantially perpendicular to an axis of said body portion, to a second position.