Lots of our modern medicines come from plants. Before proper clinical studies were a thing, humans (us) used common knowledge given over to us from our ancestors about plants we'd learned worked to treat our problems.
When the science became a thing, it became possible to figure out which exact plant and which part of the plant worked as the treatment. Then, we started to synthesize and lower the amount of side effects that once happened as a result of people eating plants whole as medicine.
Digoxin is a cool example. Originally, people would eat the foxglove plant to treat all sorts of problems related to the heart, and it took a few hundred (!) years for science to understand what part of it to use to treat heart conditions. Now, doctors use digitalis, which comes from foxglove, to directly treat heart conditions.
Painkillers (opiates yay!) are another—they originally come from the opium poppy plant. Codeine and morphine are now synthesized based off the original molecule.
Quinine, from the cinchona tree from South America, is still used to treat malaria. Also rad, quinine is used in drinks such as tonic water for its taste.
As we've written about before, aspirin was first used when people chewed the bark of the willow tree to reduce pain. Now, we synthesize the chemical that works to reduce pain—aspirin's scientific name is salicylic acid.