(Reuters) - Merck & Co Inc said on Wednesday it expects data from a large study designed to assess the effectiveness of its cholesterol drug Vytorin to be available in 2014, instead of 2013.

The closely watched, 18,000-patient trial, called IMPROVE- IT, was designed to assess whether Merck's drug Zetia, when combined with its older cholesterol drug Zocor, is more effective in reducing heart attacks and strokes than Zocor alone.

Zocor, known also as simvastatin, is a statin similar to Pfizer Inc's Lipitor. Zetia cuts cholesterol using a different method. The study aims to show that Zetia confers an additional benefit over Zocor alone.

Merck's drug Vytorin combines Zetia and Zocor in a single pill and is the product being used in the trial.

Zetia and Vytorin are among Merck's biggest-selling products, generating combined sales last year of $4.3 billion. Doubts about their effectiveness in cutting heart attacks has slowed growth.

The hope among investors is that positive results from IMPROVE-IT will reinvigorate those sales.

Questions about the combination treatment arose after data from a previous trial known as ENHANCE found that Vytorin was no more effective in reducing plaque in the carotid artery than Zocor alone and did not answer the question of whether the addition of Zetia further cut the rate of heart attacks.

And while Vytorin did a better job of reducing "bad" LDL cholesterol, it did not bring about significant changes in other key measures in patients with a hereditary form of high cholesterol known as familial hypercholesterolemia.

The IMPROVE-IT trial is scheduled to end once 5,250 clinical events, such as heart attacks and strokes, have been reported.

In March of this year, the trial's independent Data Safety Monitoring Board completed a second interim efficacy analysis and recommended no change to the study design. It said it planned to review the data again in approximately nine months.

Merck said that review has now been scheduled for March 2013. Based on the current rate at which events are being reported, the company said it anticipates the targeted number of events will be reached in 2014.

Merck's shares were up 0.14 percent at $44.06 in afternoon trading on the New York Stock Exchange.

(Reporting by Bill Berkrot and Toni Clarke; editing by Gerald E. McCormick, Jeffrey Benkoe and Sofina Mirza-Reid)