An assembly(113) for a drug delivery device (200, 202) is presented. The assembly comprises a housing (102)and a piston rod (204) having a longitudinal axis, a distal end and a proximal end. The piston rod (204) further comprises a piston rod interaction feature(206), and a piston rod guide (224)being movable with respect to the piston rod (204), the piston rod guide (224) further comprising a guide interaction feature(232)corresponding to the piston rod interaction feature (206, 226), the piston rod guide (224) being movable with respect to the piston rod (204) to switch between a first state and a second state of the assembly(113), the second state being different from the first state. The assembly is configured such that, in a first state of the assembly, the piston rod guide (224) is in a first axial position relative to the piston rod (204) in which the piston rod interaction feature (206, 226) is arranged to interact with the guide interaction feature (232) to prevent proximal movement of the piston rod (204) with respect to the housing (102), and wherein, in a second state of the assembly(113)being different from the first state, the piston rod guide (224) is in a second axial position relative to the piston rod (204), the second axial position being different from the first axial position, wherein, in the second axial position, the piston rod interaction feature (206, 226) and the guide interaction feature (206, 226) are arranged such that proximal movement of the piston rod (204) with respect to the housing(102) is allowed.
