For a fantastic discussion about the Byzantine process for generic drug approval, I recommend a recent episode of EconTalk[0]. The guest wrote a book called "Drug Wars: How Big Pharma Raises Prices and Keeps Generics off the Market"[1]. When up to half a billion dollars of revenue is at stake, the drug companies have incredible incentives to delay or prevent generics coming to market. Believe or not, they often simply refuse to give a sample of the old drug to the generic company. No sample to compare, no FDA approval. It is illegal to dispense a drug without a prescription, and the sample is not for a patient, so it can take some maneuvers to get a legal sample. Or the drug company can change a tiny feature of the drug, re-patent the new drug, and then destroy/withhold all the old drug. No old drug sample, no FDA approval of an "identical" generic. The drug companies will contract with a generic company that never, actually gets to production (wink, wink). Those contracts are finally being understood by judges, so now they are going to a second level of contract obfuscation that is even harder to notice, but still keeps the generic off the market.
[0]http://www.econtalk.org/archives/2017/06/robin_feldman_o.htm...
[1]https://www.amazon.com/Drug-Wars-Pharma-Raises-Generics/dp/1...