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).
