When I was in the Army (Ft. Drum) my roommate use to do heroin in the bathroom. I didn't know until we had a random drug test one morning and the 1st Sgt came looking for him. We told him he got selected for the piss test. 1st Sgt said "Yes, but he didn't show up".
We found out he got in his truck and went ghost. He was AWOL for 28 days
They ended up kicking him out
I definitely never had an issue with the users they were always the easiest to discharge. Most of the other ones required a shyt ton of paperwork and appointments. A druggie was easy. Long term awols weren’t an issue neither it’s those ones who leave and come back quickly that are the issues.