The present invention relates to stopper (1) for an injector for delivery of a pharmaceutical composition and to an injector with the stopper. The stopper has a stopper body (2) with an actuating surface (3) opposite an outlet surface (4), an axial length between the actuating surface and the outlet surface, and a transverse diameter, which stopper body defines an access diameter, the stopper at an axial location from the actuating surface comprising a deformable sealing element (5) surrounding the stopper body and having an outer diameter, which is larger than the transverse diameter, which deformable sealing element is made from a thermoplastic elastomer (TPE) and has an axial extension in the range of 5% and 95% of the axial length of the stopper body, and the stopper comprising a cavity (6) at the axial location of the deformable sealing element, the cavity having a lateral extension larger than the access diameter of the stopper body.
