Generic drugs are considered identical to brand-name drugs by the FDA, but variations in manufacturing processes and excipients can exist.

“so a generic is a version of a brand-name drug that is made either after a brand-name drug has gone off patent it's no longer legally protected or if the generic company has successfully challenged the brand patent in court and then the FDA gives them permission to make a generic so the FDA recognizes just to take a step back if you make two batches of drugs in the same manufacturing plant using the same ingredients those two batches will be a little bit different there'll be a little variation so now if you take a generic where a company has reverse engineered the brand sort of broken it down in a lab tried to figure out how to put it together maybe with a different set of manufacturing steps but then concocted it maybe they've used different excipient switch is additional ingredients but you have the sort of central molecule which has already been tested for safety and efficacy then they have to present data to the FDA the FDA req there's going to be differences so they've provided a range which is a range of the absorption of the drug into the blood and basically the generic has to present data to the FDA showing through their testing that they've hit within that range and that the drug is essentially clinically equivalent in the body which is called bioequivalent”