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Teva Pharmaceuticals has stopped shipping its generic version of the popular antidepressant Wellbutrin XL 300 after the U.S. Food and Drug Administration said the pill does not work properly.

And in a second setback for the drug maker, Teva yesterday suspended testing of its generic version of Rituxan, a $7-billion a year drug in advanced clinical trials. The company is considering how best to meet the requirements from the FDA and European regulators.

Teva's Wall Street-traded shares were down about 0.5% at $40.59 at 8 P.M. Israel time Thursday after falling about 1% the day before.

Teva was asked by the FDA to withdraw the antidepressant Budeprion XL 300 after new testing showed the drug releases its key ingredient faster than the original drug Wellbutrin XL 300, which is made by GlaxoSmithKline. A spokeswoman for Teva told the Associated Press that the company stopped shipping the drug last Thursday.

The FDA's move contradicts its previous update on the issue in 2008, when regulators said the drugs were essentially the same. That review came after hundreds of patients complained that Teva's drug did not work as well or caused side effects like headaches, anxiety and insomnia.

The website ConsumerLab, a small privately held company that independently tests drugs and nutrition products, published an analysis of the two drugs five years ago, indicating Budeprion XL released 34% of its active ingredient in just two hours, compared with 8% for the original drug.

Generic drugs approved by the FDA are required to be chemically equivalent to the original products, producing the same medical effects in patients.

The FDA said its initial, incorrect ruling was based on tests of the 150-milligram version of Budeprion, under the assumption that those results would also apply to the 300-milligram version. The agency said that approach "is no longer appropriate."

Meanwhile, the suspension of the clinical testing of the copycat version of Rituxan puts Teva and its partner in developing the drug, the Swiss company Lonza, behind rivals Sandoz and Celltrion. The latter two are forging ahead with Phase III tests for oncological indications and are expected to announce the results of their tests in the first quarter of 2013.

Rituxan is used in the treatment of cancers such as chronic lymphocytic leukemia and non-Hodgkin lymphoma, as well as rheumatoid arthritis. It is Roche's best-selling drug, with sales of about $7.1 billion in the first half of 2012.

Rituxan is a monoclonal antibody that is all but impossible to duplicate without precise information from the original maker. Roche, the Swiss company that developed the brand-name Rituxan, secured global patent protection until the end of 2013 except in the United States, where the drug is protected from competition until 2018.

Regarding Rituxan's use for indications of rheumatoid arthritis, Teva is up against the South Korean Kobol Samsung, which is partnering with Quintiles Transnational in development.

Teva said in response that the company's "first priority is to our patients and providing them with quality medicines. Upon receiving the communication from the U.S. Food and Drug Administration, Teva has immediately ceased shipment of Impax's 300 mgBudeprion XL. This update to the FDA’s guidance affects the bioequivalence rating of the product and does not reflect any safety issue.

"Teva is firmly committed to the development of biosimilars. However, given the changes in the regulatory and competitive environment, Teva (through its joint venture with Lonza) is evaluating the path forward for rituximab. Therefore, no phase III trial have been commenced and no patients were enrolled to it. Further updates of Teva's detailed review of its R&D portfolio will be highlighted during the Company’s investor day presentation scheduled for December 11, 2012.

The Associated Press contributed to this report.