The present invention relates to a medical injection system comprising an injection device (100) and a dose capturing unit (20). The injection device (100) incorporates expelling means (101) for setting and expelling a set dose amount of a drug from a reservoir. The expelling means (101) further comprises mechanical energy storage unit adapted to drive the expelling movement and a movable dosing member (350). In one form the dose capturing unit (20) is configured for being releasably attached to the injection device (100). The dose capturing unit (20) comprises a dose sensor (20d) configured to capture data relating to the movement of the dosing member (350) during dose expelling. When the dose capturing unit (20) is attached to the injection device (100), the systems forms a speed limiting means (F, M) to limit the speed of the dosing member (350) during dose expelling so that a force that counteracts the force exerted by the mechanical energy storage unit is generated.
