To gain FDA approval, the manufacturer of a generic drug must show that its product has the same active ingredient as the brand product and is “bioequivalent” to the brand product (meaning that the drug reaches the part of the body the drug is intended to treat at the same rate and to the same extent as the brand product).
The "80% to 125%" criteria used for determining bioequivalence are part of a statistical tool used in testing of drugs. These limits do not represent the absolute difference between drug products. In order for a generic to meet the FDA's criteria, the test results for the drug must be at, or very close to, 100 percent equivalent to the brand name in absorption into the bloodstream.
In a large FDA study that looked at more than 2,000 tests of generic drugs, the average difference in absorption into the body between the generic and the brand name was 3.5%. This amount of difference is expected and acceptable, whether for one batch of brand name drug tested against another batch of the same brand, or for a generic tested against a brand name drug. A difference of 3.5% is not expected to impact patient health. It is important to note that some variability can and does occur during manufacturing, for both brand name and generic drugs.