My favorite is Barry Marshall. He thought there was a connection between bacteria and ulcers, which was an unpopular opinion at the time. So he intentionally drank the offending bacteria, got sick as expected, and then people believed him.
More here, including (which I didn't know until now) cardiac catheterization.
I'm sure better sources exist but https://www.discovermagazine.com/health/these-five-doctors-experimented-on-themselves-and-made-big-breakthroughs
Our first was a girl. Second was a boy. Third will be a vasectomy.