Lots of doctors from reputable places eg Mayo on X. Weighing in on things with media coverage like whether cholesterol drugs are a good treatment option. Possibly they're countering quakery from charlatans with good, measured opinion. That opinion could also be influenced by paying for their conference attendance, or just plain cash. Undisclosed, such payments would be far from obvious and radically change one's opinion on how convincing their case is or isn't.

The new weight loss drugs. Are they good? How serious are the side effects? How often are there problems? Is the risk low and benefits high? Are they good for cancer? Are they good to help alcoholism? Are they scary? Should I take them?

Much legit debate there and I'd rather know who is being paid for their opinion faced with the choice when trying to weigh probability in the face of imperfect information.

The Doctor could conceivably be a passionate professional who wants to share health information with a wider audience. You know, as opposed to a shithead shill. The fact that being a corporate mouthpiece is apparently the default assumed state is a condemnation of our current medical standards.

It's easy to imagine a stereotypical "When my patients find their pancreas swimming around in their fish tank, I recommend ACME Pancrease De-marination forceps". But the more important bit is the massive grey area. Let's say your kid plays with Bob's kid on their little league team, you've added each other on Facebook, you put out a post asking if anyone has any advice about your fish tank dwelling pancreas and bob chimes in because he's a doctor. Do you assume Bob is being paid then?