A medicament delivery device comprising : a housing , a plunger rod received by the housing , which plunger rod is rotatable in a first direction relative to the housing and linearly displaceable thereby, an activation button arrangement linearly displaceable from a first position to a second position to thereby rotate the plunger rod in the first direction, a second direction rotation preventer having an annular outer member that is rotationally fixed relative to the housing and an annular inner member partially received by the outer member , wherein the outer member and the inner member are arranged to receive the plunger rod and wherein the inner member is rotationally interlocked with the plunger rod , wherein the outer member and the inner member are structured to allow the inner member to rotate in the first direction relative to the outer member and to prevent the inner member from rotating in a second direction opposite to the first direction, and a recording unit comprising: a first sensor arranged to detect rotation of the inner member , and processing circuitry configured to generate dose expulsion data in response to detection of rotation of the inner member , and wherein the inner member has as an outer periphery provided with a plurality of recesses , wherein the first sensor comprises an electromechanical switch in physical contact with the outer periphery of the inner member , and wherein the electromechanical switch is arranged to be actuated by interaction with the recesses , and wherein the processing circuitry is configured to determine the number of times the electromechanical switch is moved in and out from the recesses during one drug administration, and wherein the processing circuitry is configured to determine an injected dose based on the determined number of times the electromechanical switch is moved in and out from the recesses . a housing , a plunger rod received by the housing , which plunger rod is rotatable in a first direction relative to the housing and linearly displaceable thereby, an activation button arrangement linearly displaceable from a first position to a second position to thereby rotate the plunger rod in the first direction, a second direction rotation preventer having an annular outer member that is rotationally fixed relative to the housing and an annular inner member partially received by the outer member , wherein the outer member and the inner member are arranged to receive the plunger rod and wherein the inner member is rotationally interlocked with the plunger rod , wherein the outer member and the inner member are structured to allow the inner member to rotate in the first direction relative to the outer member and to prevent the inner member from rotating in a second direction opposite to the first direction, and a recording unit comprising: a first sensor arranged to detect rotation of the inner member , and processing circuitry configured to generate dose expulsion data in response to detection of rotation of the inner member , and a first sensor arranged to detect rotation of the inner member , and processing circuitry configured to generate dose expulsion data in response to detection of rotation of the inner member , and wherein the inner member has as an outer periphery provided with a plurality of recesses , wherein the first sensor comprises an electromechanical switch in physical contact with the outer periphery of the inner member , and wherein the electromechanical switch is arranged to be actuated by interaction with the recesses , and wherein the processing circuitry is configured to determine the number of times the electromechanical switch is moved in and out from the recesses during one drug administration, and wherein the processing circuitry is configured to determine an injected dose based on the determined number of times the electromechanical switch is moved in and out from the recesses . <EOS>