I Have been type 2 for over 15 years but after all medication stopped working, I have now been prescribed Insulin (Tresiba) an had my first instructional appointment yesterday with my Diabetes nurse where she watched me give myself an injection with the pen, as she will be titrating my dose, I am starting with only 8 units. After the appointment, I realised that I had put the pen I used back into the box with the other 4 pens and now I dont know which pen I opened and used. I was told that I should discard the opened pen after 28 days, but now I dont know which pen this is. Because it was only 8 units used, there isnt a decrease of liquid in the clear window and seems to be no other indication of which one has been opened. Do I need to throw them all away and ask the nurse for a new pack of five pens? Or keep them all for just 28 days? thank you in advance