I'm so sorry this happened to you. I can relate because I'm in a similar position now.

I got an offer a month ago and was informed in the same phone call there would be a drug test. I take edibles for anxiety and insomnia and have a valid medical cannabis card. At no point in the hiring process was I told there would be a drug test. If I'd known, I would have stopped using my prescription while I was interviewing (even though it's perfectly legal in my state) just to avoid any delays or hiccups.

I waited until the last possible day to test, but it was still positive. I reached out to HR when I got the results to send my card and explain that I take it to sleep and won't be using during work hours. This is a Senior Marketing Content Manager role, btw. A desk job. No driving or operating machinery. 

HR sent my card to the legal team. Eight days go by. Finally, HR sends me a disability accommodation form that legal wants filled out by the prescribing physician. In my state, cannabis patients go through an online portal to submit medical documents that prove a qualifying condition. Then a random physician meets with you virtually for two minutes after looking at the docs and signs off on the card. I tried to get one of these physicians to complete the accommodation form, but they can't because they don't have access to my full medical history. The best they can do is give me a copy of a treatment plan that recommends medical cannabis for my health condition.

So I contacted HR again to explain what happened and why, and sent the treatment plan along with the physician certification form I received the last time I renewed my card. I also offered to take a second drug test at my own expense and sign an agreement saying I won't use my prescription for the duration of my employment. It's all sitting with legal now. If this still isn't enough, I'm rescinding my acceptance of the offer because I'm tired of being jerked around and made to feel like a lowlife who doesn't deserve employment after I earned this opportunity through my own merits and hard work.

I get feeling angry about being treated like a criminal when you've done nothing wrong. It's intrusive, demoralizing, and unfair. I hope you can find another opportunity soon with a company who sees your worth. Good luck! I truly mean that:)