Ethically speaking, we should not be experimenting on humans, even with their explicit consent. It's not allowed by any credible review board (such as the IRB) and in many countries you can be held legally liable for doing experiments on humans.
With that being said, there have been exceptions to this, in that in some countries we allow unproven treatments to be given to terminal patients (patients who are going to die from a condition). We also generally don't have repercussions for folks who experiment on themselves because they are perhaps the only people capable of truly weighing the pros and cons, of not being mislead by figures of authority (although I do think there is merit of discussing this with regards to being influenced by peers), and they are the only ones for which consent cannot be misconstrued.
N=1 self studies are somewhat common historically though, right? Albert Hofmann synthesized LSD in his lab and took the first documented LSD trip. More recently, I seem to recall that one of the Modena founders took their Covid vax the moment they synthesized it in early 2020 (having trouble finding a citation on that, though).
i feel you are confused. internal review boards, of which there are many, regularly allow human trials. they are necessary for the fda's approval as well. there are tons of ways for patients to access ethically reviewed experimental treatments, but terminal patients are extremely likely to. you are correct that experimenting on onesself is often far less troubling though
Maybe the same as with drugs in sports: Self-experimentation can be an expedient shortcut, and scientists are often very competitive people. If results obtained through self-experimentation are rewarded, many scientists would be tempted to do it. Contrary to doping in sport, however, in science you need to at least do something different each time for it to be publication-worthy. That institutes a big skill floor and considerable risk, so I think a self-experimentation epidemic is unlikely. Generally I still think self-experimentation is good, precisely because it's such a shortcut.