Often times when you look at herbal medicines and such that have been used for thousands of years seem pretty off the wall.
I don’t really have any concrete examples offhand, but you hear about the stuff all the time. The natives of an area will know that a certain flower crushed up will cure a headache, or if you crush certain seeds and boil it in with a certain root it’ll break a fever.
Or, for that matter, who found out that smoking Marijuana got you high? Did people just smoke hundreds of different weeds and leafs and see what happens?
How did people come up with this stuff? Was it just a matter of trial and error? “Hey, this guy is sick… let’s try mixing… uh… that leaf with that flower and see what happens” and eventually find something that seems to work?
Does anyone still try to find new cures? Perhaps if you boil pine cones in rat’s blood and then stick them up your ass you’ll get a really cool buzz.
Now I’ve never monitored a hunter gatherer or early agricultural society for centuries to watch their development of herbal medicine, but I think it’s safe to guess that the medicines were discovered through trial and error and accident. That and “he’ll be well in a day or two anyway, but I’ll give him this harmless stuff to seem useful to the tribe”.
Pharmaceutical companies are actively hunting new stuff, not only by testing known herbal remedies for actual usefulness and active components, but by extracting components from any plant they can get their hands on and then trying to guess if it could be usefull. Your local herbalist in the Amazonas probably only tries something new if it seems you might die and he’s desperate.