I agree. Treat as though you don't know it's medical. If they're legit, they shouldn't mind providing you proof. If you can somewhat pinpoint the resident, schedule some inspections and see if you can detect the exact location. Also, I'm not sure how your lease reads but it's not legal in any capacity in my state and we have a crime-free addendum that includes anything drug-related that residents to have to sign. So with that specifically included in our lease, that gives us the right to inspect, detect and even do an emergency eviction if necessary. Like the others are stating, you have to think about the disruption of the other neighbors as well. Now if it's legal medically, that's going to be tricky and I'd consult an attorney as well. You might want to add a new section in your lease addressing this very issue moving into 2019 and beyond. Good luck!