The FDA rates generic drugs in two areas: chemistry/manufacturing and bioavailability/bioequivalence. Generic drug manufacturers must prove to the FDA that the generic drug is consistent in potency, stability and sterility to the brand drug. The generic drug must also show that it has the same absorption rate from the gastrointestinal tract to the bloodstream as the brand drug. Generic drugs are rated based on a two-letter code (for example AA, AB, and B_). A generic drug that is AB rated (most generics are AB rated) has had differences in bioavailability/ bioequivalence from the innovator of the drug that has been resolved to the satisfaction of the FDA.
