Hirschowitz, like a lot of fellows and young researchers in science, first tested the device on himself in February 1957. He managed to control his own gag reflex, passed through his esophagus and looked around in his own stomach. He then scoped a patient indicated on the slide as patient #2. A most remarkable aspect of this discovery was that it was not supported by grants and was carried out by a GI fellow, and two physicists who worked very hard to create this incredible advance.
The same talk also notes the Australian researcher who discovered the role of Helicobacter pylori in curing stomach ulcers tested the culture on himself:
He stated that his halitosis was so bad, that his wife told him that he had to sleep on the couch. After the infection was established he treated himself with the anti-Helicobacter therapy and completely recovered.
https://blogs.harvard.edu/lamont/2020/01/30/advances-gastroe...