A stopper includes a main body portion defining an open rearward end configured to receive the plunger rod, an engagement portion provided along an inner circumference of the main body portion configured to engage at least a top portion of the plunger rod, a closed front end forming a flexible roof, and a first perimetrical skirt extending around an outer circumference of the main body portion toward the closed front end of the main body portion. When fluid pressure is increased inside the syringe barrel during an injection, the flexible roof expands in a radial direction toward an inner wall of the syringe barrel, the first perimetrical skirt is forced against the inner wall of the syringe barrel, and an engagement between the plunger rod and the engagement portion forces the main body portion to expand in the radial direction toward the inner wall of the syringe barrel.