As a physician for more than three decades and a Molecular Biologist before that, I know a lot of science and medicine (I also keep up with the literature.)
I find that people hold on tightly to their beliefs about health matters, and many don't know how to search or evaluate the literature to inform their beliefs. I don't mind this; I don't know anything about an enormous number of things, but I like to learn. An extreme example of this is anti-vaxxers, but there are much 'softer' positions that are equally incorrect.
When people find out socially that I'm a doctor, they want to discuss Big Pharma, conspiracy theories (e.g. doctors know the cure for cancer but are withholding it to make money), want to know if I'm a "Lyme-literate" doctor (Southcentral PA is a Lyme hotspot, so it's a very real concern here), or want to discuss a particular disease and what causes it. I don't take offense at these approaches, because I hope to help people understand, and sometimes I actually succeed.
But sometimes they go nowhere, or worse. For example, if I cite a study on dogs, they'll reply, "But dogs aren't people!" without knowing that dogs are an incredibly good model for human disease.
Yesterday I was speaking with a breeder about a disease my dog has. She doesn't trust vets (they're in it for the money) and said it wasn't possible that my dog had (x), that I shouldn't have (a radiological study to determine the extent of x), that I should just change the dog's diet. When I said I've treated the same disease in people and this test is necessary, she shouted, "BUT DOGS AREN'T PEOPLE!!!"
I told her that she was right, that dogs aren't people, but they are an incredibly important model for human disease, and that many breakthroughs in medicine were thanks to the study of the same disease occurring in dogs.
She then snorted that doctors are in it for the money.
I like being social, I like discussions, and sometimes the discussions are fruitful and satisfying. Sometimes they're only frustrating. I don't like being rude, so I engage in these discussions not knowing which direction they'll take.
I have never said, "I don't discuss medicine off-duty". Or "I don't discuss science." I would feel I was being rude and antisocial. If I just change the topic, it's clear I simply disagree.
How can I break this cycle?
Edited to add (in response to comments): I rarely tell people I'm just meeting that I'm a doctor. If I'm asked directly, I'll answer honestly. I love science and will bring it up (did you hear about [amazing breakthrough of the year]?) but I will not bring up medicine unless it's with friends who know I'm a doctor and it's something funny. I address people at their level; you can't communicate effectively with a patient (or anyone else) without taking note of their own level of comprehension and using it yourself. (It's not an "I'm smarter than you" thing. I may be smarter about some things, but I guarantee I'm downright stupid when it comes to technology and a bazillion other things.) My work has been caring for people, and I care deeply about people who put their health in my hands. Can't say that extends to everyone I meet, though. I write much more formally than I speak. Finally, the dog conversation happens a lot because I used to be a breeder (Border Collies, the best dogs in the world!) and I know a lot of dog people.
[Btw, I love dogs and don't like the thought of experiments on dogs or any other animal. I'm a "flexitarian", which means I rarely eat meat. That's because I love animals, not because I think meat is unhealthy. But scientists work on dogs despite my feelings.]
Dogs have approximately the same number of genes as humans, most of them being close orthologues... Importantly, pet dogs share also the environmental conditions of their owners and are thus not only affected by genetic traits but also by “life style.” Hundreds of spontaneously occurring common canine conditions are analogous to human diseases such as diabetes, cancers, epilepsies, eye diseases and autoimmune diseases, not to mention the high numbers of rare monogenic diseases.