A fluid transfer device for use in an infusion system having a first end and a second end for coupling to an injection port of the infusion system. The device further includes at least a first member, a hollow needle attached to the first member, and a second member telescopically displaceable in relation to the first member, allowing the hollow needle to penetrate a flexible barrier member sealing the injection port thereby creating a fluid passage into the infusion system. The first end has a connecting portion for attachment to a drug bottle containing a fixed dose of a medical substance, and the second end has a flexible membrane able to be pressed against the flexible barrier member with a pressure sufficient to create a double-membrane sealing around the hollow needle.
