Meanwhile, senior lawmakers in Congress issued a demand for data on the clinical trial that indicated a cancer risk.
Vytorin, a combination of Merck's Zocor and Schering-Plough's Zetia, has been heavily promoted as a novel way to reduce cholesterol. Zocor, a statin drug, reduces the amount of cholesterol produced by the liver. Zetia limits the amount of cholesterol absorbed through the digestive system. But the combination became a focus of controversy after a study earlier this year showed it was no better at reducing the buildup of plaque in the arteries than the much cheaper generic, Zocor.
Separately on Thursday, leaders of the powerful House Energy and Commerce Committee asked the companies for extensive data on the clinical trial that indicated a possible cancer risk for Vytorin.
Merck and Schering-Plough said they would cooperate with the panel.
The companies defend the drug, saying it is effective at reducing cholesterol-its approved use.
Committee Chairman Rep. John D. Dingell and Rep. Bart Stupak, chairman of its Oversight and Investigations subcommittee, sent a letter to the chief executives of the drug companies, giving them two weeks to supply detailed information. The two congressmen, both Democrats from Michigan, are investigating drug industry safety issues and marketing practices, and have been focusing increasingly on Vytorin.
"Vytorin's effectiveness has been in doubt, and now its safety is questionable," Stupak said in a statement.
One question the congressmen want answered concerns discrepancies in the number of cancer cases reported by different researchers in the Vytorin study.
Schering-Plough spokeswoman Mary-Fran Faraji said one study report had more cancer cases because it included a longer period of patient follow-up. She added that the FDA investigation was expected.
The study was originally designed to determine whether Vytorin could help prevent a worsening of heart valve disease, but found that it did not.
Statin drugs lower the levels of LDL-cholesterol, also called 'bad' cholesterol because of its role in heart disease. Some previous studies of statins have suggested a link between low LDL levels and a higher risk of cancer. But again, others have not.
Just this week, the Journal of the American College of Cardiology published a new analysis of 15 statin studies including more than 90,000 patients that found statin users were no more likely to get cancer than people given dummy drugs.
"Nobody should avoid taking a statin because of concerns about cancer," said American Cancer Society epidemiologist Eric Jacobs.
Ironically, statins a few years ago were being studied as a possible prevention for certain cancers; those studies ultimately found no effect on cancer, good or bad.
The FDA anticipates its investigation and analysis will take about 9 months.