A drug delivery assembly comprises a housing (110), a piston (120) axially displaceable in the housing, and a variable volume reservoir (105) formed by the housing and the piston and containing a fluid first drug. The assembly further comprises an exterior fluid inlet (150) arranged in or configured to be arranged in fluid communication with the reservoir, a fluid outlet (145) arranged in or configured to be arranged in fluid communication with the reservoir, biasing means (130) configured for moving the piston forwards, and releasable retaining means (113) configured for retaining the piston in an initial position, whereby the biasing means is allowed to move the piston forwards to expel fluid drug from the reservoir when the retaining means is released.
