FDA spokesperson Sandy Walsh told Dow Jones Newswires the agency that it was also reviewing the scientific formulation of the drug, called Budeprion XL, after a consumer product testing group published results that showed key differences from the original drug.
The tests, conducted by ConsumerLab.com, came after Budeprion XL users earlier this year began complaining that the drug caused severe headaches, digestive problems, anxiety, tremors, and insomnia. Another Web site, peoplespharmacy.com, reported that more than 250 people had complained about the drug.
Budeprion XL is made by U.S.-based Impax Laboratories Inc. and sold by Israeli firm Teva Pharmaceutical Industries Ltd.
Impax Chief Financial Officer Arthur Koch said the company is studying ConsumerLab's results, but noted the drug went through a full scientific evaluation.
"Our studies were very good, and we were very satisfied with them, and by receiving that approval so was the FDA," he said, adding that Impax would cooperate with the FDA probe.
"We remain confident in our product," he said. "We're going to work through this in a professional and responsible way."
In a statement, Teva said its "number one priority is to provide safe, effective and affordable generic products to patients."
In ConsumerLab's test, the once-daily Budeprion XL released 34 percent of its active ingredient after two hours, compared to 8 percent for the original drug.
"It's been an eye-opener for everyone," said Dr. Tod Cooperman, president of ConsumerLab.com. "It makes you question whether generics are always going to be equivalent to the original product. If these things are releasing at such different rates, it's hard to believe they'd be acting the same way in your body."
Earlier Thursday, the FDA said it could not provide data to support ConsumerLab's findings.
"FDA cannot offer any examples where generics have been shown to not perform as expected. FDA has many years of experience in the review of generic drugs and has great confidence in the quality and equivalence of generic drug products," the agency said.
In 2005, Canadian firm Biovial petitioned the FDA to require rigorous testing on generic versions of Wellbutrin XL before being approved. While some saw it as a delay tactic, the company said it was to protect patients from serious risks, such as seizures.
The FDA approved the first generic versions of Wellbutrin XL in 2006.