The present invention relates to a bung (20) for drug containing cartridges (10) for use in drug delivery devices wherein the bung (20) has a distal end face (22), a proximal end face (24) and a lateral area (26), wherein the bung (20) comprises at least two different materials, a first material (30) covering the whole lateral area (26) of the bung (20) and a second material (32) which is at least partly arranged inside the bung (20), wherein the first material (30) has a larger compressibility than the second material (32) and wherein the whole lateral area (26) has a flat surface. Furthermore, it relates to a cartridge (10) and to a drug delivery device comprising a bung (20) as claimed.