Generics Drugs are Given Much Leeway in How They Perform

For a generic to be considered “bioequivalent” to the brand name drug, blood levels need only be within 80% to 125% of the original. This applies to both the maximum amount of drug that appears in the blood as well as to the average amount over time. This amount of leeway can be of great concern when drug levels must be tightly controlled. Making matters worse, if one generic is at 80% of the original, while another is at 125%, switching from the first generic to the second would result in a patient getting 53% more drug. Because the FDA does not release the differences in the bioavailability of generic drugs, patients and doctors would not suspect such an extreme difference in the products.

Bioequivalence tests are often not required for each strength of a generic. That is, the FDA may accept results for a 150 mg version of a generic in order to approve a 300 mg version, as in the case of Budeprion XL 300 (containing bupropion hydrochloride), a generic version of the brand name anti-depressant Wellbutrin XL 300.

The total amount of active drug in a generic product, such as a pill, is also permitted to vary. The generic must contain between 90% and 110% of the amount in the brand name product. After years of complaints of performance problems with generic forms of the thyroid medication levothyroxine sodium, in 2007 the FDA tightened the requirement to 95% to 105% for generic versions of this particular drug. However, manufacturers have been given until October 2009 to comply.