Connection assembly (10) for a drug delivery device that includes a tip (11) and an adapter (13), the tip (11) defining a longitudinal axis (A) and including a mounting area (21) for mounting the adapter (13) thereon, and said adapter (13) including a mounting part (26) forming a radial surface, which defines a mounting opening (27) for mounting said adapter (13) on said tip (11 )； wherein said mounting zone (21) includes at least a first part (25； 30) having an external surface that is located at a first distance from said longitudinal axis (A) and at least a second part (22), adjacent to said first part (25； 30), which has an external surface that is located at a second distance from said longitudinal axis (A), said second distance being different from said first distance, said radial surface being located after assembly of said adapter, on said first part (25； 30) and said second part (22)； said mounting opening (27) and said at least a first part (25； 30) and second part (22) are sized with respect to each other such that said radial surface of said mounting part (26) is in close contact with the external surfaces of both, said first part (25； 30) and said second part (22), said assembly part (26) and at least one of said first part (25； 30) and said second part (22) dimensioned with respect to each other such that this mounting part is compressed in a direction perpendicular to said longitudinal axis (A) when this mounting part is located on said first part (25； 30) and said second part (22), characterized in that: the mounting part (26) of the adapter (13) is made of a thermoplastic material and that the mounting area (21) is substantially grooved, said mounting area being (21) delimited by a bottom (22) and proximal and distant projections 1, and wherein said mounting area (21) comprises a projection (25) or groove (30) projecting from said bottom (22), the projection (25) or groove (30) forming the at least a first part and forming the bottom (22) the at lea
