The invention relates to an adaptor (1) for connecting a drug delivery device (100) to a needleless access device (200) having a straight external thread, the adaptor comprising engagement means for mounting said adaptor on the drug delivery device, and an internal thread intended to cooperate with said external thread (201) on a determined length L, said internal thread showing on a distal part (5b) of said length L a major diameter D1 mating that of said straight external thread, characterized in that said internal thread shows on a proximal part (5a) of said length L a major diameter D2, where D2 is strictly smaller than D1. The invention further relates to a drug delivery device comprising such an adaptor and to methods for connecting the needleless access device to the adaptor, and to the drug delivery device.
