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.

“We were stunned, to say the least,” said Dr. William E. Boden, a professor of medicine and preventive medicine at the University at Buffalo who was a trial investigator.

What is remarkable about the study is that niacin seemed to be working. Patients taking the medicine along with Zocor had higher levels of H.D.L. and lower levels of triglycerides, a fat in the blood. Despite these seeming improvements, the patients fared no better and may have done slightly worse than those taking Zocor alone. That is why the entire theory behind trying to increase H.D.L. levels in patients with heart disease may need rethinking.

The study results may be greeted as a mixed blessing by some patients. A drug many had hoped would help is now thought to be at best useless. But for many people, niacin is hard to take because it can cause flushing and headaches. Doctors have for years wheedled patients into tolerating these side effects in hopes that the medicine would save their lives. Now, they will not have to.

Dr. William M. Schreiber, a Louisville, Ky., internist, said he had stopped prescribing niacin because so many patients told him they could not abide its effects. “I’m delighted to hear that statins alone are just as good as statins and niacin,” he said.