Wow, Paula, you could be writing that message about my son. For 9 years my son convinced me that he was "only doing pot". He couldn't hold a job, was not pleasant to be around and looked like death for the most part (dark circles under his eyes, extremely thin). I believed him and also felt that "only pot" shouldn't be so bad even though the results I saw was pretty bad.
The mother of an old friend of his came and chatted to me at a market and it turned out that my son has been lying to me and that he has been doing some hard drugs. In that same week I found out that he was growing and selling magic mushrooms. It is hard to describe the devastation I felt. I realised I believed him because it made it easier to overlook what he was doing. In hindsight I saw that he must have been doing harder drugs for a very long time. He has now moved out.
Maybe he will only do pot, but whatever the drug is, if it is affecting their ability to function and be self-supportive, it is too much. I would think carefully about letting him smoke under your roof. It feels to me that if I said no more a lot earlier, maybe my son wouldn't have started with harder stuff.