Meant to respond to this earlier - I would say that in a vacuum you definitely should say something, but this may be complicated depending on where you live in the world, and, to be blunt, whether the merest hint of sympathy for drug users and their afflictions would be grounds for putting you on some kind of list which would put you at risk of state-sanctioned persecution in the future... whether this persecution takes the relatively passive form of denying you certain painkilling medications should you need them in the future, or the more actively malevolent form of monitoring your communication as a potential subversive and eventually manufacturing a reason to incarcerate you. I'm gonna presume that this latter occurrence is probably not likely, hopefully, but I have been surprised before even in my own country where we have a social healthcare system by unexpected consequences of the ingrained prejudices of potentially just 1 individual. This didn't happen to me personally, but a friend of mine confessed a long history of drug use to a doctor during either a bad comedown or possibly bad trip - mind you this is a person who was never a "problem user" in the traditional sense of the term - and was surprised to find out later by chance that there was now both a literal and metaphorical red mark against their name which no doubt would affect the kind of substances they would be prescribed in the future, should they need them.
Anyway, so that said, and with the understanding that considerations of personal safety are important and something you need to consider, if you judge the risk to your own current and future wellbeing to be low enough and within your own risk tolerance, then you should say something. I'll concede it's not black and white but generally speaking if people hold prejudiced views, and you feel able to challenge them, then you should do so. Because it might be, and often is, that they have just never had these particular ingrained prejudices challenged, they've always hung around with people who share similar views to them, or people who would disagree but think that it's probably just not worth the effort, and maybe it's not, but you won't know until you try. You shouldn't expect to be able to change anyone's mind in a single conversation, of course, but I think it's enough just to force people to think differently about something for a moment - just to plant that seed of doubt which they might forget about forever and write you off as another bleeding heart liberal junkie sympathiser or whatever their preferred vernacular... or they might think about what you've said later, maybe something else will happen that reminds them of it, maybe the ideas you've proposed to them start to take root and the next time they're dealing with someone with a substance use problem they treat them just a little more compassionately, whether consciously or not. If this is the case, or even if there is a chance this is the case, then I would say that any effort on your part to challenge an ignorant, prejudiced and distinctly discompassionate viewpoint is definitely not wasted.
I would say if you're not going to see this doctor again anyway, then you're under no moral obligation to march into his office and make a big deal out of it. But if you are going to see him again, I would not pretend like it's nothing. It doesn't have to be a confrontation of course - in fact it shouldn't be a confrontation, if you are to have any hope of having some positive impact then you should approach it in the friendliest way possible.
What I would do maybe is just say something like, "Hey, so I noticed last time we spoke you seemed kind of offended on my behalf about the mix-up with addiction counselling?" then go from there... This is a very soft approach I think and if this seems to rile them significantly then you might be in for an uphill struggle, and could be forgiven for aborting depending on how confident you're feeling about making your point - although I would suggest persisting just a little further.
You can follow up by just asking "What's up with that?", and again, go from there, or perhaps, just state that it's been on your mind, that it bothers you because there is already so much prejudice towards drug users, which they don't deserve, and you know this because X, Y and Z. You know your own reasons for wanting to challenge what in the very best estimation is a a flippant regurgitation of a prejudice that they have not thought about very much but are not used to having challenged, and at worst an expression of considered, personally held but evidently harmful beliefs.
So ultimately I would say just be honest, be friendly and open and don't write them off as ignorant, not worth the effort, backwards, whatever, even though some or all of these things might be true, you won't know until you have a proper attempt at speaking to them about it. But without being confrontational, be direct and clear about your reasons for bringing this up. If you need to rehearse your own viewpoints so that you can speak them confidently, do so - no shame in doing this, even values which we consider ourselves to hold very strongly, if we are not used to actually being called to defend them, we can catch ourselves forgetting some of the explicitly rational reasons that we believe these things - or, sometimes (maybe all the time, really) we believe things for emotional reasons and don't bother to trace back the rationality inherent to these beliefs, which we all need to do if we are to have any hope of convincing anyone else of them.
Again depending on where you live and the personal risk to yourself in doing this, you can admit to being a drug user yourself (if this is true for yourself, I'm just assuming, maybe it isn't but this goes for anyone else reading), or, more cautiously, perhaps, that you have known many drug users personally and the stereotypes do not hold. I used to always hold off on actually self-identifying as a drug user, even when discussing drug related topics with crowds that were highly likely to be drug friendly like charity events promoting decriminalisation and whatnot - it's stupid of course but I had internalised the prohibitionist narrative that says that being a drug user will impact my credibility, which I think is a hard thing to shake. But at one of these aforementioned events the founder of one of these organisations opened his talk by saying amongst other things that one of the reasons he had founded the organisation was that he was a drug user himself, which personally struck me as incredibly brave and I intend to try to emulate this bravery in any future interactions where any case I might be making for, I'll say it, pro-drug points of view might be strengthened by my own admission of being a drug user - and, of course, if I'm not endangering myself by doing so, I wouldn't do this in front of a member of law enforcement while carrying illegal substances on my person.
Slight diversion there but I think it's still directly relevant to the topic at hand - of course it isn't necessary to self-identify as a drug user and this might be explicitly inadvisable depending on where you live, and it might not even be true for you which shouldn't diminish the strength of the case you (hopefully) are going to make either way, but if this person thinks that it is truly so obvious who is a drug user and who isn't then it might shake up their worldview a bit to discover that this is not, in fact, the case.
