A supplementary device configured to be releasably attached to a drug delivery device. The supplementary device comprises a housing having a channel configured to slidably receive the drug delivery device； a first alignment feature to ensure a specific alignment of the supplementary device relative to the drug delivery device and restrict rotational movement of the supplementary device around a drug delivery device； and a second alignment feature to prevent sliding movement of the supplementary device relative to the delivery device once attached thereto. The second alignment feature comprises a moveable securing member biased towards the drug delivery device, and a release member operable to move the securing member out of engagement with the drug delivery device.
