Everybody is different.

I can’t metabolize most painkillers, they do literally nothing to me because I’m missing a liver enzyme. I could eat a handful of percacet every day for months and quit cold turkey easy, because I was never really getting anything out of to begin with.

There are countless reasons why people react differently to medication, that’s why doctors just try random shit till something seems to work.

It might be educated guesswork, but it’s still guesswork. And there’s very little effort put into why/how one thing worked over others, if it works that’s the end of it.