I think the natural engineer intuition is that if a drug is just a chemical, and generic and name-brand drugs are the same chemical, how could there possibly be any difference? Don't be duped by advertising, save the money, be rational, etc.
One interesting case where there was a real problem was with generic extended-release bupropion (an antidepressant). The time-release mechanism on one of the generics didn't work correctly, although the manufacturer managed to get it past the FDA (they actually tested only a 150mg dose, and extrapolated the results to approve the 300mg dose).
Sometimes small differences in formulations will be mostly irrelevant (oh no, your ibuprofen takes a few extra minutes to absorb, but will 99% absorb under almost all circumstances).
But other times, a new crystalline form exists that you weren’t aware of and destroys your entire ongoing production. Same molecule, but different structure that’s too stable and doesn’t dissolve effectively. See: ritonavir.
One interesting case where there was a real problem was with generic extended-release bupropion (an antidepressant). The time-release mechanism on one of the generics didn't work correctly, although the manufacturer managed to get it past the FDA (they actually tested only a 150mg dose, and extrapolated the results to approve the 300mg dose).
https://blogs.sciencemag.org/pipeline/archives/2012/10/18/th...