A spring assisted drug delivery device comprising: a housing； a piston rod having a longitudinal axis, a distal end and a proximal end that is axially movable in a distal direction relative to the housing, a mid-body fixed inside of the housing and coupled to the piston rod in such a manner that the piston rod is axially movable in a distal direction relative to the mid-body, and wherein a preloaded spring is operatively coupled to the piston rod in such a manner that the spring is released and the stored force in the spring supports driving the piston rod in the distal direction during a dose administration step.
