U.S. researchers announced Friday they would expand the size of a major study involving controversial cholesterol drug Vytorin, in order to determine its benefits.

Vytorin, marketed by a joint venture of Merck & Co. and Schering -Plough Corp., has come under fire because previous study raised questions about its effectiveness. The drug, combining a statin with a drug called Zetia did not slow progression of heart disease any more than a statin alone, preliminary results showed. These findings drove Vytorin prescriptions down 18 percent and slashed $49 billion from the drugmakers’ market value. The study, called Enhance is set to be presented Sunday at the big American College of Cardiology meeting in Chicago.

The researchers running the trial said in a statement they would expand the study to 18,000 patients to strengthen the results, which aim to show the drug prevents heart attacks and other heart problem by lowering a key measure of inflammation known as CRP. Higher levels of CRP are associated with a higher risk of heart attack. The study, originally designed for 10,000 patients, is expected to be completed in 2011.

Trial chairman Dr. Eugene Braunwald of Brigham and Women's Hospital said in a statement that expanding the trial will allow researchers to test definitively whether the additional lowering of low-density lipoprotein -- or LDL, the so-called bad cholesterol -- with Zetia will translate into additional benefits for patients, Reuters reports.

However, the move to increase the number of patients participating in the study could fuel controversy surrounding Vytorin. Its manufacturers were already criticized for making changes to the Enhance study midstream, and for attempting to change the study’s primary measure after it was completed.

They were also criticized for not releasing the Enhance results for nearly two years after its 2006 completion. But, the companies have defended their handling of the Enhance study and said the delay was caused by data-quality problems.

That did not fit the opinion of Michigan Democratic Reps. John Dingell, chairman of the Committee on Energy and Commerce, and Bart Stupak, chairman of the Subcommittee on Oversight and Investigations who said they suspect the drugmakers suppressed the data to protect their profits.

They are still investigating the way Merck and Schering-Plough dealt with the situation and asked the U.S. Food and Drug Administration for records related to television commercials promoting the drug.