Abstract: A flexible piston rod for use in cylinder ampoules comprises a tape shaped body (1) which is along its length provided with supporters (2; 6; 16) projecting from at least one side of the tape. The tape (1) has a width and the supporters a length so that sides of the tape and the free ends of the supporters abuts the inner wall of the ampoule (3) in at least three positions along the inner perimeter of the ampoule (3) and not all the abutment points lies on the same half of the circular cross section but are typically displaced about 120° from each other. The piston rod is moved by a gear engaging a rack (12) on the tape either on the side not carrying supporters or at the edge of the tape on the side carrying supporters.