Dose setting device

A drug delivery system comprising:

• a housing;
• an ampoule accommodated in the housing and having an interior with first and second ends, a piston closing the first end, which piston can be pressed into the ampoule, a membrane closing the second end, and a medicine disposed in the ampoule interior between the piston and the membrane;
• a piston rod mounted in the housing and comprising first and second parts, wherein the first part is flexible and has a first end which is maintained in contact with the piston and a second end which is deflected 180 degrees relative to the first end and includes a thread, wherein the second part includes a thread which mates with the thread on the first part so that rotation of the second part relative to the first part changes the total length of the piston rod, wherein the housing includes a guide allowing the first part to move axially but not rotationally, wherein the second part is mounted in the housing for both axial and rotational movement, wherein rotation of the second part relative to the first part causes the second part to move in a first direction away from an initial position to set the amount of a dose, and wherein the second part can be moved in said first direction away from said initial position no more than a maximum distance representing a maximum dose;
• and
• an injection mechanism comprising a push button which can be moved between a projecting position and a pressed home position, wherein the injection mechanism includes a piston rod moving element that, responsive to moving the push button from its projecting position to its pressed home position, moves in a direction opposite to said first direction from a first end position to a second end position and engages the second part during at least part of such movement to move the second part back to said initial position upon reaching said second end position, and wherein the distance between the first and second end positions is at least as great as said maximum distance, whereby the distance of movement of the push button is independent of the size of the set dose.