Merck & Co. jointly markets Vytorin and Zetia with Schering-Plough. Vytorin is a combination of the cholesterol drugs Zetia and Zocor, which was Merck's top selling drug until it lost patent protection last year.

Schering-Plough shares fell $1.06, or 3.7 percent, to close at $27.94 Wednesday. The stock has traded between $22.30 and $33.81 over the last 52 weeks. Meanwhile, Merck shares fell 68 cents to $59.72.

The House Committee on Energy and Commerce wants more information on the large, late-stage study for Vytorin. The company has delayed releasing the study data, citing the complexity of the data.

The study began in 2002, the same year Zetia was approved by the Food and Drug Administration, to determine whether adding Zetia to Zocor was effective in further reducing possible heart-attack causing plaque. Vytorin was approved in 2004, but results from the study have not yet been released.

Schering-Plough and Merck said they expect to release the data at the American College of Cardiology meeting in March.

"The trial results have been delayed, with management noting that the sheer volume of images is slowing the analysis, and critics insinuating that the delay speaks poorly of the results," said Goldman Sachs analyst James Kelly in a note to clients.

The calls for more information come amid heightened regulatory concern about safety and increased jostling for a larger share of the cholesterol drug market.

In late 2006, Pfizer Inc. ended late-stage development on its planned successor to the cholesterol drug Lipitor, or torcetrapib, over concerns it was connected to a high number of deaths. Lipitor, which commanded $13 billion in sales last year, will soon lose patent protection and is already losing market share because of generic cholesterol drugs, notably the generic version of Zocor.

Merck, meanwhile, said it plans to ask the Food and Drug Administration to approve its new cholesterol drug Cordaptive while moving ahead with late-stage development of anacetrapib.

Ultimately, the study results could have a large impact on both Schering-Plough and Merck's fundamentals, Kelly said, if it shows that adding Zetia to Zocor causes harm.