1) Place or find a medical kit on the ground
2)kit must have at least one empty syringe and one 100% full glass vial of antibiotics (actually cant remember the name of this item, i think it was antibiotics)
3)you must have free space in your inventory, I used my backpack.
4)with the medkit open on the ground, drag the syringe onto the vial
5)it should give you an option to fill the syringe, do so
6)you should now have a filled syringe in your inventory, and the vial and empty syringe should still be in the med kit on the ground.
Note: The vial does not even get used up with this bug, I was able to sit in a house with my friend and make about 40 of these, we agreed it should be reported.