May 6 (Bloomberg) — U.S. regulators plan an extensive study of blood-pressure drugs that generate almost 40 million prescriptions a year, after receiving thousands of complaints from doctors and patients.

Generics of AstraZeneca Plc's extended release Toprol XL make up about 90 percent of physicians' orders for the drug. The copycat drugs are made by Wockhardt Ltd. and Dr. Reddy's Laboratories Ltd., both of which are based in India, and Mylan Inc. and Actavis Plc in the U.S.

Complaints against the drug, known generically as metoprolol succinate, cite both a lack of effectiveness and troublesome side effects, according to a review of 3,425 adverse incident reports by Bloomberg News. While the reports generally don't identify manufacturers, the Food and Drug Administration tests will focus on whether the copies are chemically the same as the original, the funding request shows.

"The FDA will continue to monitor and review available safety and other information related to metoprolol succinate extended release products and take any further action as appropriate," said Sandy Walsh, a spokeswoman, in an e-mail.

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