The invention relates to a medical injection device comprising a container body (1) and a stopper (2) in gliding engagement within the container body (1) for expelling a fluid (3) through an opening of said container body (1), wherein: —at least the distal portion (20) of the stopper (2) that is in contact with the fluid (3) is made of a thermoplastic polyolefin, —the stopper (2) further comprises at least one O-ring (4, 4′) maintained in at least one peripheral groove (210, 210′), —said O-ring (4, 4′) is made of a butyl-type rubber; and —the cross-section of said peripheral groove (210, 210′) is designed so as to compress both axially and radially the O-ring (4) when said O-ring (4) is engaged in the peripheral groove (210) between the stopper (2) and the inner wall (10) of the container body (1).