The present invention provides a needle unit (10, 310, 410, 510) having a proximal space (13, 513) adapted to accommodate a portion of a variable volume reservoir, the needle unit (10, 310, 410, 510) comprising: a needle carrier (15, 315, 415, 515), a needle tube (21, 121, 221, 321, 421, 521) being fixed to the needle carrier (15, 315, 415, 515) and comprising a distal needle end (23, 123, 223, 323, 423, 523) for providing fluid communication to an
injection site, a needle shield (50, 350, 450, 550) carrying a sealed chamber (38, 138, 238, 338, 438, 538) for accommodating a
distal portion of the needle tube (21, 121, 221, 321, 421, 521), the sealed chamber (38, 138, 238, 338, 438, 538) being sealed distally by a penetrable self-sealing septum (39, 139, 239, 339, 439, 539), and a flow channel (24, 124, 224, 324, 424, 524) for establishing fluid communication between the sealed chamber (38, 138, 238, 338, 438, 538) and the proximal space (13, 513), wherein the needle shield (50, 350, 450, 550) and the needle carrier (15, 315, 415, 515) are capable of
relative motion between a first relative position in which the sealed chamber (38, 138, 238, 338, 438, 538) houses the distal needle end (23, 123, 223, 323, 423, 523), and a second relative position in which the distal needle end (23, 123, 223, 323, 423, 523) protrudes from the sealed chamber (38, 138, 238, 338, 438, 538) through the penetrable self-sealing septum (39, 139, 239, 339, 439, 539), and wherein the needle tube (21, 121, 221, 321, 421, 521) further comprises a side hole (25, 125, 225, 325, 425, 525), the side hole (25, 125, 225, 325, 425, 525) being in fluid communication with the flow channel (24, 124, 224, 324, 424, 524) and positioned within the sealed chamber (38, 138, 238, 338, 438, 538) when the needle shield (50, 350, 450, 550) and the needle carrier (15, 315, 415, 515) are in the second relative position.