Monitoring unit (200, 302) for a
drug delivery device, wherein the monitoring unit comprises: - a case (202, 300), - a monitoring circuit (400) arranged in the housing (202, 300) and capable of detecting and monitoring functions of the
drug delivery device when the
drug delivery device is in use, - a fastening mechanism which is provided with releasable locking elements (208, 306) for releasable fastening to the
drug delivery device, wherein the fastening mechanism has a number of flexible locking elements (208, 306) which are arranged to interact with matching locking elements on the
drug delivery device, - a mechanical interface (231, 318) arranged to interact with a matching mechanical interface located on a
drug delivery device, and - an activation switch (242, 320) which is arranged in the mechanical interface and is arranged to activate the monitoring circuit (400), wherein the drug delivery device is provided with an activator element (114) which is arranged in the mechanical interface and is operationally connected to a drive element (98, 104) of the drug delivery device, wherein, when the drug delivery device is used, the drive element (98, 104) pushes the activator element (114) in a distal direction during the
dose delivery sequence, thereby acting on the activation switch (242, 320).