Here's my initial thoughts. Your employer does have a duty to accommodate, but they are going to need proof of the need. Your forms signed by your doctor ARE proof, just as a doctor's note would be for any other situation. As you pointed out, HC is simply a registration centre. Refusal is grounds for a discrimination suit, and I would make that crystal clear. If mmj is a new issue for their workplace, they may need to draft a new policy and I would volunteer to help create that. It shows you are willing to work with them, rather than in opposition. I have had to do it, so I'm happy to help if needed.