a 1b for example the ends of the flexible outer structure 4 are thicker than the middle a proximal section 4 3 of the flexible outer structure 4 extends over the plunger 2 2 and comprises a plurality of longitudinal recesses 4 4 to allow for a resilient deformation of the proximal section 4 3 when the plunger 2 2 is pushed in the distal direction d to expel the medicament in particular the longitudinal recesses 4 4 form elastic elements in the form of arms in the flexible outer structure 4 the arms have thick ends and are thin in the middle such design with varied dimensions of the thickness of the flexible outer structure 4 supports deformation and strength of the biasing force the longitudinal recesses 4 4 have different lengths in particular one pair 4 4 1 of corresponding longitudinal recesses 4 4 has a length shorter than another pair 4 4 2 of corresponding longitudinal recesses 4 4 or vice versa such design supports the deformation and bending outwards of the flexible outer structure 4 during drug delivery two projections 6 protrude from opposite sides of the flexible outer structure 4 in a radial direction to provide a gripping means adapted to support the fingers of a user of the injection device 1 during an injection additionally the outer structure 4 provides an outer cover for the safety device s and.