The FDA's drug approval criteria are extremely strict. Ask anyone who works in pharma and they can probably name at least a couple of drugs which showed great promise but got axed by the FDA on dubious grounds.
A classic example is Aspirin. We know from over a hundred years of experience that Aspirin is a widely useful and relatively safe drug, but if it were discovered today the FDA would never allow it to market, let alone approve it for OTC sales. Aspirin's mechanism of action is far too broad and the gastrointestinal side effects alone would be considered grounds for rejection.
Perhaps if you dropped a few more specific examples would could talk about their issues instead of in generalities, instead of Aspirin? I was going to reply in my own words, but will instead yield to this retired FDA compliance officer: https://www.quora.com/Would-aspirin-be-approved-by-the-FDA-t...
A classic example is Aspirin. We know from over a hundred years of experience that Aspirin is a widely useful and relatively safe drug, but if it were discovered today the FDA would never allow it to market, let alone approve it for OTC sales. Aspirin's mechanism of action is far too broad and the gastrointestinal side effects alone would be considered grounds for rejection.