You make a black box test on several thousands (sometimes only hundreds) patients, and if patients who received the drug perform better the patients who received the placebo, then the drug is usually accepted for commercialization.
Yet one isolated patient may be subject to several comorbidities, her environment could be weird, she could ingest other drugs (or coffee, OTC vitamins or even pomelo) without having declared it. In a recent past women were not part of clinical trials because being pregnant makes them very "non-standard'.