https://www.washingtonpost.com/news/wonk/wp/2015/07/23/one-big-myth-about-medicine-we-know-how-drugs-work/

Here’s how we think we discover powerful new medicines: Scientists dig deep into biology and zero in on a molecular Achilles’ heel that could disable a devastating disease, be it cancer or an infection. They concoct experimental drugs that hit the target. Then they conduct trials to find one that is safe and effective in people.

Here’s how we actually develop a surprising number of treatments: good old-fashioned observation, trial and error, and luck. Detailed scientific understanding of how a drug works often comes, ironically enough, near the end of the process.

More trial and error has gone into Ivermectin than those covid vaccines.