Speaking as a nurse?
I'm not someone who likes to get people in trouble. So I would be thinking about the patients, and what kind of job that person had. If they were a fellow nurse, and it was their job to administer drugs, control that each patient got the right dose, etc..? That would be considerably worse than if they were, say, a regular health worker, who had no such responsibilities. In the latter case..? I would probably ignore it. But in the former case..? I would tell them: "hey. This is not okay, you know? I don't want to turn you in, or screw you over. But you need to stop doing this at work. Or else..? I'll be forced to take action. For the sake of our patients."
..Which... I would really, really hate having to be that person! But I'd feel a stronger sense of loyalty to my patients than to that one co-worker, and I would not want for anything to put them at risk. I've really got no problem with people drinking, or using other recreational drugs, at home! Or at work, if it's NOT putting anyone at risk! Like.. if they're a comedian, or a bartender, or an actor, or a waiter, or a receptionist, or whatever. But just.. it's not okay if you're a nurse, or a doctor! Just like it also wouldn't be okay if you were a pilot. Or a bus driver. Or some other career, where you were also responsible for lots of other people's safety.