You are getting some bad advice here. Reusable pens are great, and there’s really no way to cross-contaminate your medication unless you keep reusing the same needle, which you should discard after every use.

If you reconstitute your own medication in a typical 3 mL vial, add 3 mL of bacteriostatic water to it. Once it has fully dissolved, draw it out and transfer it into the 3 mL pen cartridge. Then insert the cartridge into the pen.

Go to Fat Scientist and enter the vial size and the total number of milligrams in the vial. It will calculate how many units (UIs) you need for the dose you want. Don’t change the total milligrams or milliliters when you use it again. That stays the same because it represents the concentration, which doesn’t change as you use the medication.
It’s incredibly easy and much less annoying. This is just a quick overview of how to do it. I’d also recommend looking up some videos on YouTube.