Even if the brain is not apart from the rest of the body, there is a special thing between the two: the blood-brain barrier, which is actually quite hard to cross with medications, and poses a real challange in curing brain diseases.
And even if they didn’t, the body is a tightly interconnected system. Taking an antidepressant to “selectively affect serotonin” in the brain? Whoops, turns out your gut has at least as many serotonin receptors as the brain. Taking a statin to reduce cholesterol (which it does very well but with an NNT > 200 for heart attacks and NNT > 300 for stroke)? Whoops, it causes transient global amnesia and personality changes and diabetes in a lot of people. And 40% of the US Adult population we’re on statins in the mid 2000s.
I'm not in the medical field, but try to keep up just a little bit to have some basic knowledge, but it's quite hard as often I see that Wikipedia is many years behind.
At the same time medical articles take a lot of time to read, as they are much less accessable.
2) Best I could find in 2 minutes. Will look for the studies. https://www.scientificamerican.com/article/gut-second-brain/
https://en.m.wikipedia.org/wiki/Blood%E2%80%93brain_barrier#...