May 3 (Reuters) - The U.S. Food and Drug Administration approved a Merck & Co cholesterol lowering pill that combines a generic version of Pfizer Inc's Lipitor with its own Zetia, Merck said on Friday. The new combination drug will be sold under the brand name Liptruzet and will begin shipping to wholesalers next week, the company said. Merck already sells a two-drug cholesterol fighter called Vytorin that combines Zetia, known chemically as ezetimibe, with Merck's older LDL lowering medicine Zocor. Lipitor, known chemically as atorvastatin, is considered to a more powerful statin than Zocor. Vytorin, which had sales of $1.75 billion in 2012, is currently undergoing a long-term trial to prove that it can reduce heart attacks and strokes better than Zocor (simvastatin) alone. While results of that study should definitively determine the value of Zetia to patients, the drug on its own rang up sales of $2.67 billion in 2012. "No incremental benefit of Liptruzet on cardiovascular morbidity and mortality over and above that demonstrated for atorvastatin has been established," Merck said. The new drug will be available in four doses, combining 10 milligrams of Zetia with either 10, 20, 40 or 80mg of atorvastatin.