Depends on the reason. I don't believe smoking pot should pre dispose you to bring banned from owning guns. Its no different from someone who uses alcohol.
My problem is if it is truly being users as medical therapy, what is the medical condition?
Is the guy psychotic? He shouldn't have guns
Is he suffering from chronic pain or some physical condition, sure no problem
Does he have some sort of job or marital anxiety ? Well that's the slippery slope. That's where things get interesting. Who gets to determine who is a danger to others?