A drug delivery system comprising: - a drug delivery device (2, 3) carrying at least one reservoir (40a, 40b), and - a needle unit being attachable to the drug delivery device (2, 3) in axial extension thereof and adapted to establish a flow way between the at least one reservoir (40a, 40b) and a drug delivery site, the needle unit comprising ∘ a needle structure comprising a front needle portion for entering the drug delivery site, and at least one back needle portion (13a, 13b) for insertion into the at least one reservoir (40a, 40b), and ∘ a needle shield, wherein - the needle shield and the needle structure are capable of undergoing relative axial motion between an accommodating relative position in which the needle structure is completely accommodated within the needle shield, and a protruding relative position in which the front needle portion protrudes from the needle shield, the needle shield and the needle structure being biased towards the accommodating relative position by a first bias force, - when the needle unit is attached to the drug delivery device (2, 3) the needle structure and the at least one reservoir (40a, 40b) are capable of undergoing relative axial motion between a disconnected relative position in which the at least one back needle portion (13a, 13b) is spaced apart from the at least one reservoir (40a, 40b), and a connected relative position in which the at least one back needle portion (13a, 13b) and the at least one reservoir (40a, 40b) are fluidly connected, the needle structure and the at least one reservoir (40a, 40b) being biased towards the disconnected relative position by a second bias force, and - the second bias force is greater than the first bias force, such that when an axial compressive force presses the needle unit and the drug delivery device (2, 3) together the needle shield and the needle structure reach the protruding relative position before the needle structure and the at least one reservoir (40a, 40b) reach the connected relative position, and when the compressive force is discontinued the needle shield and the needle structure reach the accommodating relative position after the needle structure and the at least one reservoir (40a, 40b) reach the disconnected relative position. - a drug delivery device (2, 3) carrying at least one reservoir (40a, 40b), and - a needle unit being attachable to the drug delivery device (2, 3) in axial extension thereof and adapted to establish a flow way between the at least one reservoir (40a, 40b) and a drug delivery site, the needle unit comprising ∘ a needle structure comprising a front needle portion for entering the drug delivery site, and at least one back needle portion (13a, 13b) for insertion into the at least one reservoir (40a, 40b), and ∘ a needle shield, ∘ a needle structure comprising a front needle portion for entering the drug delivery site, and at least one back needle portion (13a, 13b) for insertion into the at least one reservoir (40a, 40b), and ∘ a needle shield, - the needle shield and the needle structure are capable of undergoing relative axial motion between an accommodating relative position in which the needle structure is completely accommodated within the needle shield, and a protruding relative position in which the front needle portion protrudes from the needle shield, the needle shield and the needle structure being biased towards the accommodating relative position by a first bias force, - when the needle unit is attached to the drug delivery device (2, 3) the needle structure and the at least one reservoir (40a, 40b) are capable of undergoing relative axial motion between a disconnected relative position in which the at least one back needle portion (13a, 13b) is spaced apart from the at least one reservoir (40a, 40b), and a connected relative position in which the at least one back needle portion (13a, 13b) and the at least one reservoir (40a, 40b) are fluidly connected, the needle structure and the at least one reservoir (40a, 40b) being biased towards the disconnected relative position by a second bias force, and - the second bias force is greater than the first bias force, such that when an axial compressive force presses the needle unit and the drug delivery device (2, 3) together the needle shield and the needle structure reach the protruding relative position before the needle structure and the at least one reservoir (40a, 40b) reach the connected relative position, and when the compressive force is discontinued the needle shield and the needle structure reach the accommodating relative position after the needle structure and the at least one reservoir (40a, 40b) reach the disconnected relative position.