Canada

If I was going under anesthesia for a planned procedure, I would not drink or use drugs beforehand. If it was an unplanned emergency, I guess it would depend on the circumstances, but you are citing an extreme circumstance. If someone is actively high at the time they end up in the emergency department, well, that's bad luck and it might be wise to disclose since the staff will figure it out pretty darn quick anyway. Same if you are a severe alcoholic or opiate addict. That's not what I'm talking about, though. I'm talking about when doctors or nurses ask you about it as a lifestyle question.

You know as well as I do that the health care system classifies people in terms of their risk factors and then use that profile to make decisions about you. Once classified as a "drug user" in your chart, many doctors and nurses will treat you differently. They may or may not "care" from a moral perspective, and we know that they won't tell family or police, but that won't necessarily stop them from denying you necessary pain relief or deprioritizing you in triage. That's the actual concern.

There is absolutely no reason to tell a doctor if you use cannabis or engage in moderate alcohol use or occasionally use cocaine, LSD, or psylocibin. If you are prescribed a medication that has an interaction with a recreational drug, the doctor can simply tell you that. They don't need to know if you use that drug from time to time. Only you, the patient, need to know that so you can avoid the interaction. More extreme forms of drug use are a different story, of course.

Edit: Let me add one other overarching point. I think people are sick and tired of having doctors make decisions for them. I don't need a nanny. I need information about risks and benefits in order to make an informed choice. Doctors rarely do that. Instead, they decide what should be prescribed, or not prescribed, regardless of the patient's wishes. I know the reason is fear of liability, but here we are nonetheless.