The difference in blood concentration between Wellbutrin and bupropion in this study could explain the clinical difference in safety and effectiveness, Preskorn said. "If the concentration is substantially lower or higher, then your concern would be reduced efficacy or greater likelihood of off-target effects," he said.
Although it is unclear why only the generic XL in 300-mg tablets and not in 150-mg tablets, or only the Impax/Teva version of the 300-mg tablets, would lack bioequivalence, it could be because higher doses of the drug have trouble dissolving in the gastrointestinal tract, Preskorn said.
The FDA approved the generic versions of Wellbutrin XL based on the studies demonstrating bioequivalence at the lower, 150-mg dose.
Typically the FDA recommends that makers of generic drugs test the blood concentration of the drug at the highest dose and then extrapolate bioequivalence data for the lower doses based on these findings.
However, in the case of bupropion, the FDA granted a waiver to companies to test the lower dose because of concern that the higher dose could cause seizures in the volunteers, Walsh wrote.
Either way, extrapolating information about safety and efficacy from one dose is usually appropriate, said Dr. Sidney Wolfe, director of the health research group at Public Citizen, a nonprofit consumer advocacy organization based in Washington, D.C.
"For most drugs, there is such a wide difference between the amount that works and the amount that causes trouble that checking out every single dose is not necessary," he said.
However bupropion might be an exception. Ever since it entered the market in 1985, it was known there was a fine line between antidepressant effect and seizure risk, Wolfe said. The FDA knows which drugs have this type of narrow therapeutic window, and for them it might have been better to check out
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