A drive mechanism for a drug delivery device (1) comprises a housing (2), a drive member (5) and a piston rod assembly (32). The drive member (5) is configured to be axially displaced in a dose setting direction with respect to the housing (2) for setting a dose of a drug (24) and to be axially displaced in a dose delivery direction with respect to the housing (2) for delivering the set dose of the drug (24). The drive member (5) comprises a drive feature (15). The piston rod assembly (32) comprises a set of interaction surfaces (17) for mechanical interaction with the drive feature (15), with at least two interaction surfaces (17) being axially and angularly offset with respect to each other. When the drive member (5) is displaced in the dose delivery direction for delivering the set dose, the drive feature (15) mechanically interacts with one of the interaction surfaces (17) and the piston rod assembly (32) rotates with respect to the housing (2), thereby rotating and axially displacing another one of the interaction surfaces (17) from a non-interaction position into an interaction position in which the drive feature (15) may interact with the other one of the interaction surfaces (17) when the drive mechanism is actuated once more for setting and delivering a further dose. Also, a piston rod (3) is proposed.Linvention concerne un mécanisme de commande dun dispositif de distribution de médicament (1), qui comprend un boîtier (2), un élément dentraînement (5) et un ensemble tige de manœuvre (32). Lélément dentraînement (5) est conçu pour se déplacer axialement dans une direction de réglage des doses par rapport au boîtier (2) pour régler une dose dun médicament (24) et pour se déplacer axialement dans une direction de distribution des doses par rapport au boîtier (2) pour distribuer la dose réglée du médicament (24). Lélément dentraînement (5) comprend un mécanisme dentraînement (15). Lensemble tige de manœuvre (32) comprend un ensemble de surfaces dinteraction (17
