Drugs are approved for the specific uses based on extremely high-quality evidence. That evidence balances the benefits against the detectable downsides/costs/side-effects. Those downsides are also on the label.

That's almost entirely generated by the highest quality evidence generation system we could possibly have, which is RCTs. And no, pharma reps actually aren't allowed to encourage (or even talk about) off-label uses of drugs.

I get the sense that you don't know much about this space.