A drug delivery device comprising - a blocking element and - a piston rod having an interaction feature which is adapted and arranged to mechanically cooperate with the blocking element , wherein, for delivering a dose of a drug , the piston rod is arranged and configured to be moved from an initial position to an end position with respect to the blocking element , and wherein an operating noise is provided to the user when the interaction feature mechanically cooperates with the blocking element while moving the piston rod from the initial position to the end position for dose delivery, wherein the interaction feature comprises at least one indentation and at least one elevation , wherein the interaction feature is provided along an outer surface of the piston rod , wherein the device comprises a plurality of interaction features , wherein the elevations and the indentations of succeeding interaction features are arranged in an alternating fashion along the piston rod , wherein succeeding interaction features comprise an equal axial dimension, wherein, for delivering a dose, only one of the interaction features mechanically cooperates with the blocking element , wherein the drug delivery device is a fixed dose device, characterised in that each interaction feature is adapted to homogenize the operating noise so as to prevent the operating noise from being interpreted by the user as signalling the end of the dose delivery operation, and in that each interaction feature comprises only one elevation and only one indentation , wherein a height difference between the elevation and the indentation is between 0.1 mm and 0.3 mm. - a blocking element and - a piston rod having an interaction feature which is adapted and arranged to mechanically cooperate with the blocking element , wherein, for delivering a dose of a drug , the piston rod is arranged and configured to be moved from an initial position to an end position with respect to the blocking element , and wherein an operating noise is provided to the user when the interaction feature mechanically cooperates with the blocking element while moving the piston rod from the initial position to the end position for dose delivery, wherein the interaction feature comprises at least one indentation and at least one elevation , wherein the interaction feature is provided along an outer surface of the piston rod , wherein the device comprises a plurality of interaction features , wherein the elevations and the indentations of succeeding interaction features are arranged in an alternating fashion along the piston rod , wherein succeeding interaction features comprise an equal axial dimension, wherein, for delivering a dose, only one of the interaction features mechanically cooperates with the blocking element , wherein the drug delivery device is a fixed dose device, characterised in that each interaction feature is adapted to homogenize the operating noise so as to prevent the operating noise from being interpreted by the user as signalling the end of the dose delivery operation, and in that each interaction feature comprises only one elevation and only one indentation , wherein a height difference between the elevation and the indentation is between 0.1 mm and 0.3 mm. <EOS>