[techrat]

>How do you know what% users have side effects

This information is typically provided in the published drug facts for each medication.

https://pubmed.ncbi.nlm.nih.gov/2686432/

https://www.rxlist.com/cipro-drug.htm#dosage

Studies provide further info.

> or what doseage that occurs at or what causes this reaction in the subpopulation?

It's through trials they discover the incidence of side effects and appropriate dosages for maximum therapeutic benefit.

> You don't and just wave that off, that's why it's flippant.

Nobody is waving anything off when providing perspective.