A resettable drive mechanism for a drug delivery device (1) comprises a piston rod assembly (32) and an interaction member (23). The piston rod assembly (32) comprises a guide track (18) running along the piston rod assembly (32), the guide track (18) comprising at least one delivery section (18A) and at least one reset section (18B). The reset section (18B) is arranged to continue the delivery section (18A). The delivery section (18A) comprises at least one blocking means (19). The interaction member (23) is configured to cooperate with the guide track (18). For delivering a dose of a drug (24), the piston rod assembly (32) is axially displaceable in a dose delivery direction away from an axial starting position, the interaction member (23) cooperating with the delivery section (18A) and the blocking means (19) passing the interaction member (23) when the piston rod assembly (32) is displaced in the dose delivery direction. Axial displacement of the piston rod assembly (32) towards the axial starting position is prevented by mechanical interaction of the blocking means (19) and the interaction member (23) when the interaction member (23) cooperates with the delivery section (18A). For resetting the drug delivery device (1), the interaction member (23) is configured to cooperate with the reset section (18B), the piston rod assembly (32) being axially displaced towards the axial starting position, the blocking means (19) being offset from the interaction member (23) and passing the axial position of the interaction member (23) when the piston rod assembly (32) is axially displaced towards the axial starting position. Also, a piston rod assembly (32) is proposed.Linvention porte sur un mécanisme dentraînement à réarmement pour un dispositif dadministration de médicament (1), lequel mécanisme comprend un ensemble tige de piston (32) et un élément dinteraction (23). Lensemble tige de piston (32) comprend une piste de guidage (18) qui court le long de lensemble tige de p
