A supplementary device configured to be releasably attached to an injection device includes at least one wireless communication unit and at least one sensor, wherein the supplementary device is configured to activate the at least one sensor in response to receipt via the at least one wireless communication unit of a wireless communication from an external device, following activation, to use the at least one sensor to detect the start of an injection by the injection device, and to communicate, via the at least one wireless communication unit, the start of the injection to the external device.