The present invention relates to piston for use in an injector comprising a cylinder having a longitudinal axis and an inner wall, which piston has a deformable sealing element with a convex surface, which deformable sealing element when the piston is inserted in the cylinder abuts the inner wall of the cylinder at an abutting interface and seals an annular gap between the piston and the inner wall of the cylinder, the abutting interface and the deformable sealing element having axial dimensions parallel with the longitudinal axis, characterised in that the ratio between the axial dimension of the abutting interface and the axial dimension of the deformable sealing element is in the range between 0.01 and 0.2. In other aspects the invention relates to an injector comprising the piston and to the use in a disposable syringe of the piston. The piston can prevent refilling of a syringe.
