A drug delivery device comprising: a casing having an opening (110ο), the casing configured to contain a drug storage container including a dose delivery member such that at least an insertion portion of the dose delivery member is configured to extend through the opening (110ο) in the casing; a needle guard disposed adjacent to the opening (110ο) and movable relative to the casing between an extended position and a retracted position where the needle guard retracts into the casing, the insertion portion of the dose delivery member being surrounded by the needle guard in the extended position and the insertion portion of the dose delivery member being at least partially exposed when the needle guard is in the retracted position; an interference arrangement for providing a selected threshold of resistance to movement of the needle guard from the extended position to the retracted position during insertion of the dose delivery member into body tissue at an injection site, the interference arrangement having a first member (154 1) associated with an interior surface of the casing, and a second member (154 2) extending from the needle guard, the first and second members engaging one another to retain the needle guard in the extended position, one of the first and second members moving if the selected threshold of resistance is exceeded to allow the members to slide past one another and to allow the needle guard to move into the retracted position when the device is pressed toward the injection site during insertion of the dose delivery member; wherein the first member (154 1) comprises a detent member and the second member (154 2) comprises a detent follower member, and the detent follower member including a flexible arm (256, 356) that applies a resistive bias force to the detent follower member against the detent member during movement of the needle guard from the extended position to the retracted position, characterised in that the detent member includes a circumferential bead. a casing having an opening (110ο), the casing configured to contain a drug storage container including a dose delivery member such that at least an insertion portion of the dose delivery member is configured to extend through the opening (110ο) in the casing; a needle guard disposed adjacent to the opening (110ο) and movable relative to the casing between an extended position and a retracted position where the needle guard retracts into the casing, the insertion portion of the dose delivery member being surrounded by the needle guard in the extended position and the insertion portion of the dose delivery member being at least partially exposed when the needle guard is in the retracted position; an interference arrangement for providing a selected threshold of resistance to movement of the needle guard from the extended position to the retracted position during insertion of the dose delivery member into body tissue at an injection site, the interference arrangement having a first member (154 1) associated with an interior surface of the casing, and a second member (154 2) extending from the needle guard, the first and second members engaging one another to retain the needle guard in the extended position, one of the first and second members moving if the selected threshold of resistance is exceeded to allow the members to slide past one another and to allow the needle guard to move into the retracted position when the device is pressed toward the injection site during insertion of the dose delivery member; wherein the first member (154 1) comprises a detent member and the second member (154 2) comprises a detent follower member, and the detent follower member including a flexible arm (256, 356) that applies a resistive bias force to the detent follower member against the detent member during movement of the needle guard from the extended position to the retracted position,