I had always been told that the active ingredients in generic medications had to be exactly the same as their brand name counterparts; that the only differences allowed were in the inactive ingredients, such as binders. But as I began to hear from more and more people who insisted that the generic of a particular medication was not as effective for them as the brand name, I decided it was time to dig deeper.
As it turns out, brand name and generic medications aren't necessarily exactly the same. The differences and the reason we're often told they are equilivant are tied up in the definitions of three terms – bioequivalence, therapeutic equivalence and bioavailability.
Bioequivalence means that two drugs given to the same patient in the same dosage regimen will result in equivalent concentrations of the drug in plasma and tissues. Generic drugs are required to be bioequivalent to the original brand name drug.
Therapeutic equivalence means that two drugs given to the same patient in the same dosage regimen will have the same therapeutic and adverse effects. Bioequivalent medications are expected to be therapeutically equivalent, however, sometimes they are not. Often this is only discovered after long-term treatment when patients who have done well on one forumlation are switched to another.
Bioavailability is the rate at which a drug is absorbed or becomes available to the body at its site of action. This is where the differences can occur. Current regulations permit the bioavailability of a generic drug to be plus or minus 20% of the original brand name drug.
A 20% difference in bioavailability may or may not be significant depending upon the medication. For drugs that have a wide index of tolerence, 20% may not be particularly significant; however, for drugs that have a very narrow therapeutic band, it can make a big difference. And what if a patient taking a generic medication that is plus 20% bioavailability is switched to another generic of the same drug that happens to be minus 20% bioavailability? That's a huge difference mathematically, and I would think it could make a noticeable difference therapeutically as well.
So what should you do if you receive a generic medication that doesn't seem to be as effective as the brand name? Talk to your pharmacist and ask whether other generics of that drug are available from different manufacturers. There's usually a major price difference between brand name and generic medications, so it's beneficial to try to find a generic that works for you. If you're not able to find a generic of a particular drug that is effective for you, you'll need to discuss it with your doctor and request that she specify no generic substitutions on your prescription.
Mossinghoff, Gerald J. (1999).Overview of the Hatch-Waxman Act and Its Impact on the Drug Developement Process. Food and Drug Law Journal, Vol. 54, 187-194.
Food and Drug Administration, (March, 2003). Bioavailability and Bioequivalence Studies for Orally Administered Drug Products — General Considerations . Guide for Industry, 1-23.