OKLAHOMA CITY -- A controversial cholesterol-lowering drug marketed by Merck ( MRK) and Schering-Plough ( SGP) has now caught the attention of the U.S. Food and Drug Administration. In a surprise move on Friday, the agency announced plans to review a recent study of FDA-approved Vytorin. Merck and Schering-Plough count Vytorin, along with a sister drug known as Zetia, among their hottest sellers. Shares of Schering-Plough fell 5.7% on the news, while Merck slipped 3.6%. Earlier in the day, Merck and Schering-Plough were aiming to prove that they have nothing to hide. With a key drug franchise at stake, the companies on Friday released a detailed timeline of a drug trial that was shrouded in secrecy for years. The negative results of that long-awaited trial, popularly known as ENHANCE, came out last week and have caused headaches for the companies ever since. The trial suggested that Vytorin may have no greater health benefits than older Zocor. Vytorin is made of both generic Zocor and newer LDL-lowering Zetia. In the study, however, Vytorin failed to reduce plaque -- which can lead to heart attacks -- any more than did Zocor alone. Critics have accused Merck and Schering-Plough of sitting on ENHANCE's unfavorable results so that they could maximize sales of Vytorin and Zetia ahead of an imminent downturn. In a strongly worded statement issued on Friday, however, both companies adamantly claimed otherwise. "While the ENHANCE trial was time-consuming and took longer than originally anticipated to complete, our companies acted with integrity and good faith in connection with the trial," they said. "We stand behind Vytorin and Zetia and stand behind our science that has brought these cholesterol-lowering medications to millions of people around the world." That number appears to be falling, however. Doctors wrote 9.5% fewer prescriptions for Vytorin last week, The Wall Street Journal recently reported, and 12% fewer for stand-alone Zetia. That decline came as the companies, under pressure from Congress, scaled back direct-to-consumer marketing of their most popular cholesterol drug. "I am pleased that our investigation prompted these companies to pull advertising for Vytorin," Rep. John Dingell, chairman of the House Committee on Energy and Commerce, said on Wednesday. "However, our investigation is far from over -- and stopping this advertising by no means excuses these companies' behavior." Meanwhile, the Congressional committee has expanded its probe to include prominent outside supporters of the companies' popular drug. Notably, the committee is now demanding information from both the American College of Cardiology and the American Heart Association. The committee suspects that both organizations, which have recently emerged with public support for Vytorin, could be influenced by financial conflicts. The AHA counts Vytorin's makers as major sponsors of its Web site, the committee points out. And the ACC relies on Merck to finance fellowships for young doctors. "Given the AHA's and ACC's recent statements on Vytorin and the ENHANCE study, our committee is interested to learn what financial arrangements exist between the marketers of Vytorin and these two organizations," Dingell stated on Thursday. "The public places great trust in the official views of the AHA and ACC, so it is important to verify that these views have not been compromised by a financial relationship with the pharmaceutical industry." The committee also continues to demand answers from the drugmakers themselves. It wants to know whether the companies knew the study's results ahead of time -- and if their leaders sold stock based on that knowledge. It questions changes and delays in the study as well. In response, Merck and Schering-Plough have released a detailed timeline dating back to the first discussions of ENHANCE seven years ago. That timeline reveals a number of challenges encountered during the complex study -- which involved reading some 40,000 ultrasound scans -- and helps explain some of the delays.