A refillable reservoir comprising a piston assembly for filling and/or dispensing fluids contained in the reservoir, said refillable reservoir comprising: (a) a cylinder defining a reservoir for containing a fluid (b) a piston body axially slideable in said reservoir (c) a piston rod releasably fixed to the piston body for actuating the piston body, said characterized in that a first locking mechanism is provided between the piston body and the cylinder a second locking mechanism is provided between the piston body and the piston rod.
