One of the two winners of the Nobel Prize in medicine from 2005 (Barry Marshall) gave himself ulcers just to prove he understood causation and the cure. He and his partner (Robin Warren) in the research won the prize for that treatment.
I was told a story about a neuroscience study that involved paralyzing the lead author with curare and manually manipulating his eyeball to study his perception as a result of the manipulation; I was told it was never repeated, although a quick Google to find a reference revealed at least one similar study (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3232712/), so I guess it's more popular than it used to be.
Another famous one was Werner Forssmann, the first doctor to perform a cardiac catheterization, who performed the operation on himself first (then walked over to the X-ray room to get a picture to prove he succeeded).