My daughter was taking Pristiq, an extended-release, once a day pill. Once the generic for it became available, of course that's what the insurance wanted to pay for. No problem, I've always been of the opinion that drugs are drugs, and that the generic is exactly the same as the name brand.
But for this specific generic anyway, whatever they used to control the release of the drug didn't seem to work the same way. Instead of a nice steady 24 hour dose of the drug, it released very quickly, so within an hour she was extremely tired (a known side-effect, though she never experienced it with the name brand), and several hours later she would be angry/agitated/anxious/sad, all the things that the drug was supposed to help with. So it seemed like she got a big dose all at once, and then it wore off and didn't help the rest of the day.
Her doctor agreed and said that he had heard of that happening with other generics. So while the drug may be exactly the same, the "packaging" may not be, and that can have some very adverse effects. Fortunately we were able to get her back on the name brand drug, without the weird effect of the generic.