From the man who smuggled deadly parasite eggs into the US by injecting them before ejaculating them out upon his return to the doctor who saved lives by pumping hydrogen gas into his own anus, history is replete with heroes of medicine who went the extra mile for their work and conducted experiments on themselves.
There are also, as you'd expect, countless examples of people who went the extra mile but hadn't taken the initial step of knowing what the hell they were talking about. Hence, 19th-century American doctor Stubbins Ffirth chugged the vomit of patients with yellow fever to prove it wasn't infectious, even though it was.
One particularly enjoyable (if that's the right word – and it is) example of this came from Scotland in the 1700s.
John Hunter was born to a poor family in 1728. At the encouragement of his obstetrician brother William, he moved to London in 1748. He studied anatomy in William's dissection rooms and learned surgery on the job at Chelsea Hospital under the watch of William Cheselden. Back in those days, becoming somebody who chopped up live humans was very much a "give it a try and see how it goes" affair, apparently.
Despite not completing any courses at university, nor becoming a doctor of medicine (something that was common for surgeons at the time), he went on to become a respected anatomist and surgeon, eventually serving as surgeon to King George III. He also became infamous for taking the yellowish penile discharge from a man he believed to be infected with gonorrhea and rubbing it into cuts he had created on his own penis.
What could lead a man to do this, other than boredom and access to said fluids, you ask? Hunter was convinced that gonorrhea and syphilis were two forms of the same disease. At the time, both diseases were known as "the pox", despite different and distinct symptoms. Some were beginning to believe – correctly – that the two forms of pox were actually different diseases. Hunter, however, thought that the difference in symptoms was a result of differences in the tissues infected. What's more, he was so confident he was willing to bet his own penis that he was correct.
In 1767, a patient presented to Hunter with what to most eyes would look like yellow penile discharge, but to Hunter looked like an awful lot like opportunity. He took some of that goo, and put it into two puncture wounds he had created "[one] on the glans, the other on the prepuce". His notes show that within a couple of days, he began to experience pain, redness, and itchiness on his penis, before going on to develop other classic (and uncomfortable) symptoms of gonorrhea.
Later, to his absolute delight, he went on to develop symptoms of syphilis – horrendous sores on his penis. As you can imagine, he was ecstatic. He believed he had proved that the two forms of pox were actually the same disease. For fifty years, many were convinced of his discovery and thought that they would be able to prevent syphilis from developing by simply burning off gonorrhea sores with mercury. Which was incorrect.
In reality, the reason why he had developed both forms of "pox" (really gonorrhea and syphilis) was that the patient whose penis pus he had partaken of was infected with both gonorrhea and syphilis. He hadn't proved anything, nor advanced medical science – he had merely sat down and rubbed two painful venereal diseases into his own penis for absolutely no reason whatsoever, other than setting back research for around half a century.
It's worth noting that some historians believe that Hunter used somebody else as a subject for the experiment, though the majority do not. It's also worth adding that he died in 1793 of an aortic aneurysm – possibly caused by tertiary syphilis which was found during his autopsy – still believing he was right.