Dammit!

[Oh_hunnihunni]

"Natural" would be to chew willow bark. "Medicine" takes the active ingredient (salicylic acid) and puts it in to convenient pills (Aspirin, Disprin, etc) that are standardised and exclude contaminants like possum pee. There is evidence that this is safe for most people and helps mitigate the painful effects of inflammation. This evidence has come from trials (research) and is not "proof", just "evidence". Future research could change the way we view this as we learn more and/or do different trials that might provide a new result.

Science is ongoing and is not about proof, just evidence. At any given time we go with the best evidence we have, and in today's world we are very lucky that the results from most of the trials are available to the majority of people on the computer. Now laymen just have to learn to search for credible information and read, understand and interpret it. In the meantime, we rely on professionals (eg doctors) to advise us.

But it isn't willow in those pills. It is a laboratory constructed chemical compound that mimics salicylic acid.
To obtain enough from willow we would need vast acreages of those trees. Easier to analyse the source compound and reconstruct it.
Which does not mean willow doesn't still work to reduce fever and mute pain.
It's just easier to take a pill. Easier too to overdose, lol.