An assembly for a drug delivery device (200, 202) comprising: - a housing; - a piston rod having a longitudinal axis, a distal end and a proximal end, the piston rod further comprising a piston rod interaction feature, and - a piston rod guide being movable with respect to the piston rod, the piston rod guide further comprising a guide interaction feature corresponding to the piston rod interaction feature, the piston rod guide being movable with respect to the piston rod to switch between a first state and a second state of the assembly, the second state being different from the first state, - wherein the assembly is configured such that, in the first state of the assembly, the piston rod guide is in a first axial position relative to the piston rod in which the piston rod interaction feature is arranged to interact with the guide interaction feature to prevent proximal movement of the piston rod with respect to the housing, and wherein, in the second state of the assembly, the piston rod guide is in a second axial position relative to the piston rod, the second axial position being different from the first axial position, wherein, in the second axial position, the piston rod interaction feature and the guide interaction feature are arranged such that proximal movement of the piston rod with respect to the housing is allowed, wherein the assembly comprises a spring element (230, 310), and wherein, in the first state of the assembly, the spring element (230, 310) tends to move the piston rod guide from the first axial position to the second axial position, characterized in that a) the spring element is an integral part of the piston rod guide, wherein the assembly is configured such that, in the first state of the assembly, the spring element tends to move the guide interaction feature from a first radial position to a second radial position, or b) the piston rod guide is elastically deformable and the spring element is a part separate from the piston rod guide, wherein, in the first axial position of the piston rod guide, the piston rod guide is elastically deformed, and, in the second axial position of the piston rod guide, the piston rod guide is relaxed or less deformed, at least as compared to the first axial position. - a housing; - a piston rod having a longitudinal axis, a distal end and a proximal end, the piston rod further comprising a piston rod interaction feature, and - a piston rod guide being movable with respect to the piston rod, the piston rod guide further comprising a guide interaction feature corresponding to the piston rod interaction feature, the piston rod guide being movable with respect to the piston rod to switch between a first state and a second state of the assembly, the second state being different from the first state, - wherein the assembly is configured such that, in the first state of the assembly, the piston rod guide is in a first axial position relative to the piston rod in which the piston rod interaction feature is arranged to interact with the guide interaction feature to prevent proximal movement of the piston rod with respect to the housing, and wherein, in the second state of the assembly, the piston rod guide is in a second axial position relative to the piston rod, the second axial position being different from the first axial position, wherein, in the second axial position, the piston rod interaction feature and the guide interaction feature are arranged such that proximal movement of the piston rod with respect to the housing is allowed, wherein the assembly comprises a spring element (230, 310), and wherein, in the first state of the assembly, the spring element (230, 310) tends to move the piston rod guide from the first axial position to the second axial position, characterized in that a) the spring element is an integral part of the piston rod guide, wherein the assembly is configured such that, in the first state of the assembly, the spring element tends to move the guide interaction feature from a first radial position to a second radial position, or b) the piston rod guide is elastically deformable and the spring element is a part separate from the piston rod guide, wherein, in the first axial position of the piston rod guide, the piston rod guide is elastically deformed, and, in the second axial position of the piston rod guide, the piston rod guide is relaxed or less deformed, at least as compared to the first axial position. a) the spring element is an integral part of the piston rod guide, wherein the assembly is configured such that, in the first state of the assembly, the spring element tends to move the guide interaction feature from a first radial position to a second radial position, or b) the piston rod guide is elastically deformable and the spring element is a part separate from the piston rod guide, wherein, in the first axial position of the piston rod guide, the piston rod guide is elastically deformed, and, in the second axial position of the piston rod guide, the piston rod guide is relaxed or less deformed, at least as compared to the first axial position.