Drive mechanism for a drug delivery device

A drive mechanism of a drug delivery device for dispensing of a dose of a medicament, comprising:

• a housing of substantially cylindrical geometry having a long axis extending in an axial direction, the housing having a distal portion and a proximal portion,
• a nut axially and rotationally fixed to an inner surface of the distal portion of the housing;
• an at least axially displaceable piston rod adapted to become operably engaged with a piston of a cartridge containing the medicament for displacing the piston in a distal direction, where the piston rod and the nut are configured such that the piston rod moves axially in the distal direction relative to the nut during dose delivery,
• two diametrically opposed dose buttons protruding from a lateral side wall of the housing at a location defining a transition between the distal and proximal housing portions when the buttons are in an activation position, where at least one dose button is radially displaceable with respect to the housing by an externally applied depressing force for transferring a respective driving force to the piston rod,
• an activation button is located at a proximal end of the proximal housing portion and is axially displaceable relative to both the housing and the two dose buttons, where the activation button is configured to cause the two dose buttons to move radially from an end position to the activation position when the activation button is moved distally, and
• a coupling located adjacent the nut operably engaged with the dose buttons and with the piston rod, where the coupling is configured to transfer the radially inwardly directed displacement of the at least one dose button into a distally directed axial displacement of the piston rod, characterized in that the coupling comprises a drive member threadedly engaged with the piston rod or with a drive sleeve.