Pen-type drug injection device and electronic add-on monitoring module for|monitoring and logging dose setting and administration

A system comprising:

• an injection device comprising a drive screw that moves during medicament delivery, wherein the drive screw has markings formed on a surface thereof, wherein the drive screw and markings are arranged to move axially during medicament delivery;
• and
• a supplemental device comprising:
• a housing with a mating unit configured to attach the supplemental device to the injection device;
• a processor;
• and
• a quantity determiner for determining a quantity of medicament that has been delivered;
• wherein the quantity determiner comprises a sensor arranged to detect both rotation and linear advancement of the drive screw during medicament delivery,
• wherein a housing of the injection device includes an optically transparent window to allow the drive screw to be viewed by the supplemental device,
• wherein the sensor is arranged to detect an extent of axial movement of the markings via the optically transparent window, and
• wherein the processor is arranged to determine an axial position of the drive screw based on the extent of axial movement of the markings detected by the sensor.