However, it was positive for a prescription medication that had been prescribed to me.

Are you thinking of "prescriptive easement"?

Just today they arrested hundreds over a billion dollar prescription drug scam.

Just last week the tenant got arrested in front of the house (allegedly on narcotic & gun possession) & now my client really wants them out. 

Can someone sell their legally obtained opioid prescription?

Sounds like prescriptions for when 3% and 43% is exceeded mainly.

Yeah I suppose I'll have to ask, I think I have some good evidence that they were living there, lots of letter, prescription drugs, kid's report cards with the address. 

.- You can require documentation (a prescription) from a licensed medical professional documenting the need for an ESA. 

The older man with a good steady job, and an old narcotics conviction from 20 yrs ago, a number of Hispanic families with steady jobs, and who understand that you're supposed to pay your rent every month, an older Hispanic gentleman who is paying about 80% of market rent (we inherited him when we bought the building) and is so grateful that he pays BEFORE the first of the month, a very well organized AA woman with kids and a job and Section 8. 

It also states that I do not have to store any personal property besides prescription drugs which I have to store for 7 days.