WASHINGTON — Lowering bad cholesterol levels reduces heart attack risks, and researchers have long hoped that raising good cholesterol would help, too. Surprising results from a large government study announced on Thursday suggest that this hope may be misplaced.

The study could change the way doctors treat millions of patients with heart disease. Common wisdom has been that such patients should take a statin drug like Lipitor or Zocor to lower bad cholesterol and, in many cases, the vitamin niacin to raise their good cholesterol. But in the trial, niacin provided no benefit over simple statin therapy.

The results are part of a string of studies that suggest that what doctors thought they knew about cholesterol may be wrong. Studies that track patients over time have for decades shown that patients with higher levels of high-density lipoproteins (H.D.L., or good cholesterol) tend to live longer and have fewer heart problems than those with lower levels of this cholesterol.

Not surprisingly, doctors thought that if they could raise H.D.L. levels, their patients would benefit. So far, that assumption is not panning out. Nobody knows why.

In 2006, Pfizer halted development of a drug that raised good cholesterol levels after studies showed that the medicine increased the risks of death. And on Thursday, government scientists announced that Niaspan, an extended release form of niacin, not only did not provide any protection against heart attacks when taken with Zocor in patients with heart disease but also slightly increased their risk of stroke.

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The study gives no comfort to other drug makers, many of which have been trying to come up with new drugs to raise levels of good cholesterol or otherwise lower heart attack risks. Statins and other drugs have proven so effective in treating heart disease that improvements are proving very tough to find.

The study is also bad news for the F.D.A., which heavily relies on laboratory results to decide whether to approve drugs.

“This study shows that approving drugs and allowing them to stay on the market on the basis of how they affect lipids and other biomarkers is not good policy,” said Dr. Steven Nissen, chairman of cardiovascular medicine at the Cleveland Clinic. “It’s time to have a new regulatory approach.”

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