A fluid transfer device (100) for use in an infusion system, which device exhibits a first end (101) and a second end (102) for coupling to an injection port of the infusion system. The device (100) further includes at least a first member (105), a hollow needle (106) attached to the first member, and a second member (107) which is telescopically displaceable in relation to the first member (105) and allows the hollow needle (106) to penetrate a flexible barrier member sealing the injection port in order to create a fluid passage into the infusion system. The first end (101) exhibits a connecting portion (109) for attachment to a drug bottle containing a fixed dose of a medical substance, and the second end (102) exhibits a flexible membrane (111) intended to be pressed against the flexible barrier member with a pressure sufficient to create a double-membrane sealing around the hollow needle (106).
