Injection monitoring module removably mountable to a proximal extremity of an injection pen comprising a pen body, a proximally located dose setting wheel, and an injection activator, the dose setting wheel rotating about a central longitudinal axis of the pen during dose setting, the injection monitoring module comprising a hollow main body coaxially mounted on, and engaging in co-rotation with the dose setting wheel, the main body comprising a longitudinal bore having a proximal and distal extremities, and a central longitudinal axis; one or more magnets located on or within the main body; an injection monitoring system comprising at least one magnetic sensor movable in translation along the central axis within the bore, from a first monitoring position, to a second monitoring position; the injection monitoring module further comprising a rotational stop means preventing rotational movement of the monitoring system about the central axis during dose selection.