"This discrepancy in dosage could render the drug less effective in treating depression and could explain the side effects we were hearing about," Graedon said.
When results of the trials became available recently, the agency sent both Teva and Impax communications asking them to voluntarily withdraw the drug from the market. Both companies have complied with the request.
FDA spokesperson Sandy Walsh stressed that this is not a recall, which is typically done when a drug is unsafe.
"This is a voluntary market withdraw by the company for a drug that may not work well for some people. It is one type of generic Wellbutrin XL in the 300mg strength only, made by Teva. This does not impact the other manufacturers or the 150 mg strength pills," Wash said.
The agency estimates that only about 2 percent of people taking a 300 mg dose of a generic version of the antidepressant take the Teva pill. They have asked other manufacturers of generic 300 mg dose versions of Wellbutrin to conduct their own studies and submit their data no later than March 2013.
But is this voluntary recall just the tip of the iceberg? Does it mean there could be other generics in wide use that are either unsafe or ineffective? Dr. Harold Koenig, associate professor of psychiatry and medicine at Duke University said he doesn't believe this is the case.
"Most generics work just fine and are as effective as the name brand. And they are a lot cheaper. So you cannot say, based on what happened with this one drug, that we should throw out all generics," he said.
However, Koenig said he thought the FDA's actions were significant. "It might open up a can of worms if this is the first time the FDA took their own look at the effectiveness of generic versus name brand. It could lead to a lot of other drugs being studied," he said.
For his part, Graedon said he is happy with the result even if it was a long time in coming.
"It took us five years and an awful lot of arm wrestling to help the FDA make this decision, but this is really a historic moment. If you think of prescription drug use in America, generics account for about 75 percent of them, and until yesterday the FDA has maintained they are identical to brand names."