Merck & Co Inc said on Thursday a major trial of its experimental Tredaptive medicine to raise "good" HDL cholesterol showed it did not do a better job at preventing heart attacks, deaths or strokes than traditional statin drugs that lower "bad" LDL cholesterol.

Merck shares fell 3.5 percent in morning trading, on the failure of one of its most closely watched experimental drugs.

Tredaptive combines an extended release form of niacin, a nutrient that has been used for decades to raise HDL cholesterol, with a drug called laropiprant meant to reduce the incidence of facial flushing that is a side effect of niacin therapy.

Merck said it no longer plans to seek regulatory approval for the drug in the United States and is recommending that providers not start new patients on it. The medicine was approved in the European Union in 2008, but U.S. regulators were unwilling to approve it until Merck conducted the costly long-term study to better assess its safety and effectiveness.

The U.S. Food and Drug Administration has been especially concerned about potential heart risks from laropiprant.

The failed study, called HPS2-THRIVE, had enrolled more than 25,000 patients over more than three years.

It is the latest high-profile study that has called into question the heart-protective value of niacin.

Merck said Tredaptive had sales of about $13 million for the first three quarters of 2012, in the 40 countries where it is already sold. Had the drug been successful in the HPS2-THRIVE study, brokerage Cowen and Co predicted its annual global sales would have jumped to $300 million by 2016.

(Reporting By Caroline Humer and Ransdell Pierson; Editing by Gerald E. McCormick)