An electronic logging unit adapted to be housed in a drug-filled cartridge having an axially displaceable piston and an outer cavity formed between the piston and the cartridge proximal opening, the logging unit comprising a general axis, a first distal portion adapted to engage the cartridge piston, and a second proximal portion adapted to engage a rotating element having a rotational axis corresponding to the general axis. The unit is provided with sensor means adapted to detect the amount of relative rotation between the first and second portions, and storage means adapted to store data representing detected amounts of relative rotation.