The drug maker Merck announced on Thursday that a combination of niacin and another medicine failed to protect against heart attacks and strokes in a large clinical trial, and that the company would no longer pursue approval of the combination drug in the United States.

The trial, which followed more than 25,000 patients over four years, also found a statistically significant increase in the number of patients who suffered serious harm, although the company said those adverse events were not fatal. The patients studied were all taking statins, a class of drugs commonly used to lower bad cholesterol.

The results are likely to further tarnish the reputation of niacin, a drug that is often prescribed to raise the levels of good cholesterol in patients at risk for heart disease but that was found not to prevent heart attacks in a government study last year.

Merck’s drug, called Tredaptive, combined an extended-release version of niacin with laropiprant, a medicine intended to reduce facial flushing in patients, one of niacin’s more inconvenient side effects. Tredaptive is sold in about 40 countries, including in Europe, but it failed to win approval in 2008 in the United States.