A spring assisted drug delivery device (80, 90) comprising: a housing (4); a piston rod (22) having a longitudinal axis, a distal end and a proximal end that is axially movable in a distal direction relative to the housing (4), a mid-body (20) fixed inside of the housing (4) and coupled to the piston rod (22) in such a manner that the piston rod (22) is axially movable in a distal direction relative to the mid-body (20), and wherein a preloaded spring (82, 92) is operatively coupled to the piston rod (22) in such a manner that the spring (82, 92) is released and the stored force in the spring (82, 92) supports driving the piston rod (22) in the distal direction during a dose administration step.