"The FDA certifies the entire formulation, which includes everything from the binding/filler chemicals, anti-caking agents, the pill coatings, and even the ink used to mark the pill (which is why imprinting is very popular)."

Sometimes the FDA certifies a dosage of a generic based on a different dosage. This caused a problem not long ago when the dosage they assumed was fine didn't dissolve right, hence the absorbed dosage varied and was sometimes way too low. To add insult to injury, it was a psychotropic drug, so everyone assumed patients complaining were "just imagining things" for a long time.