I verbally accepted a summer internship offer and expected a urine drug test to be required. However, the official offer states the drug test is a hair test, which goes back roughly three months. I have used THC-containing products a few times over that period, not so much that I wouldn't pass a urine test but enough that I expect to fail a hair test.
My test will probably be within the next month. As a backup plan, I have obtained a medical certification approving my use from my state's department of health (a "green card"). As I understand it, a company could potentially rescind an offer after a positive drug test for THC metabolites, even if the person has medical approval. This could potentially be considered discrimination, and there have been lawsuits over this (clarified in the comments). I believe the case law is still developing (in my state at least) and no outcome is certain. I would rather avoid a complicated legal dispute if possible. All I want to do is work the job I was offered.
A friend of mine pointed out that the employer might not even be testing for THC products. It might be a long shot, but it would certainly help my stress if I knew.
Edit: My attempt to provide context may have obscured the question. Feel free to ignore my specific situation and answer the question itself (which may perhaps help others in the future).
The question: Is it reasonable to ask what substances are being tested for? What's the likelihood of getting an answer, and could trying have any negative repercussions?
Side question, not the main point: Additionally, I'd be very appreciative of any insight on whether it would be a better move to mention my green card up front or only in the case of a positive test result.