Historically, they have been scientistic method to test things out depending on the technology available and how far the knowledge of a domain went. This was always perform by scientist, or whatever the equivalent of a scholar name at the time. At the same time, there is also people who want to tell you how to cure yourself based on how their neighbor got better once.

Every plant have an effect. You are eating them after all. But the ones with too strong effect to be consumed outside medication are often famous for that, or for being toxic. If you don't have a strong medical conterindication, you should not fell more that mild positive or negative effect when consuming a eatable plant in reasonable quantity. And for seeing real effect, that's a doctor or nutritionniste job. You can't do anything at home without playing with your health.