Unless you are sitting right next to your roommate inhaling the smoke, you won't get stoned and you won't test positive.

As far as medicinal marijuana, it is called Marinol and comes in 10mg tablets. It is prescribed for cancer patients that are experiencing extreme nausea and loss of appetite. I have never heard of it prescribed in a smoking form. I think your roommate might be full of $h!t that telling you it is prescribed.