Why? The issue appears to be generic equivalency. Some people reported having headaches, anxiety and other ailments. And the agency is 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, according to the Associated Press.

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 Impax Laboratories and sold by Teva Pharmaceutical. Impax declined to comment and Teva representatives did not immediately return calls to the AP.

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,” Tod Cooperman, president of ConsumerLab.com, tells the AP. “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.” The FDA, however, couldn’t 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 told the AP.

In 2005, Canadian firm Biovail 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 drugmaker said it was to protect patients from serious risks, such as seizures. The FDA approved the first generic versions of Wellbutrin XL in 2006.