A drive mechanism of a drug delivery device is presented having, a piston rod to operably engage with a piston of a cartridge containing a medicament, a first functional component and a second functional component whose position or orientation relative to each other is unambiguously correlated to a predefined axial position of the piston rod, wherein the first and/or the second functional component comprise at least one engaging portion to inseparably and to adhesively engage the first and the second functional component with each other for rendering the drive mechanism inoperable when the predefined axial position of the piston rod has been reached.
