Steven L. Zweibel has been taking a statin drug to lower his cholesterol for seven years. It has worked, and he has suffered no problems or side effects.

But, like a lot of patients taking these drugs, he is perplexed by new guidelines on preventing heart disease and stroke — despite the fact that he is the director of cardiac electrophysiology at Hartford Hospital.

"I am very happy to be on Zocor," said Zweibel, 47, referring to the statin he takes. "But now the real question in my head is whether I need to be on it."

The new guidelines, released Tuesday by the American College of Cardiology and the American Heart Association, represent a remarkable and sudden departure from decades of advice on preventing cardiovascular disease.

According to the new advice, doctors should not put most people on cholesterol-lowering medications like statins based on cholesterol levels alone. And, despite decades of being urged to do so, patients need not monitor their cholesterol once they start taking medication. The guidelines do not even set target levels for LDL, the so-called bad cholesterol.

Doctors are also being told to stop adding other cholesterol-lowering drugs to statins, because those drugs have not been proved to reduce the risk of heart attacks and strokes.

For patients and doctors alike, all of this amounts to a surprising, and at times baffling, change in perspective.

The guidelines tell doctors and patients to use a new online risk calculator to determine whether they need treatment. Some patients, like Nancy Hayward, 60, of Sacramento, who started taking a statin in 2007 because her LDL was mildly elevated, do not qualify because they have no other risk factors for heart trouble.

Zweibel tried to use the calculator to assess his own risk of heart attack or stroke. But it requires a person’s cholesterol measurement, and Zweibel has no idea what his is without the statin.

He could, and says he probably will, stop taking the drug for a couple of months and then have his cholesterol measured. But what then? Should he stop taking the drug if the calculator says his 10-year risk of a heart attack or stroke is less than 7.5 percent, the new cutoff point for treatment?

He worries about letting his cholesterol drift up. There is dementia in his family, and cardiovascular risk factors are also risk factors for dementia.

"I wonder if Zocor could help" prevent dementia, he said. "It’s a very tough decision."

By taking a statin diligently, Roland Paul, a 76-year old corporate lawyer in Greenwich, Conn., managed to reduce his LDL level to an almost unheard-of 55 milligrams per deciliter, a feat of which he is proud. "My doctor thought it was gangbusters," Paul said.

But the guidelines committee now says there is no evidence that he is better off with that number than with a higher one. Paul said he was no longer as concerned about keeping his cholesterol so low.

The new approach poses challenges to clinicians, too. Doctors often use LDL levels as motivators to keep patients on statins. Many expect that the task will be more difficult without regular monitoring.

Dr. Barron H. Lerner, an internist and professor of medicine at New York University, gives patients a printout with their LDL levels circled before and after they start taking statins.

"It is really helpful to have some kind of results to show people," Lerner said. "I will predict 100 percent that I will have some patients who say, ‘If you are not going to check the LDL level and you cannot tell me the statin is working, then I am not going to take it.’"

Indeed, monitoring LDL levels has been an ingrained part of preventing heart attacks for decades.

"The terminology that keeps coming to mind is ‘leap of faith,’" Lerner said. "You have to trust your doctor and the people who did the studies that they are correct that you don’t have to check LDL levels."

Patients and doctors striving for low numbers are now being told that they should regard taking a statin as they might regard taking aspirin to reduce their heart attack risk: a pill a day, with no monitoring required. This advice has left some cardiologists wondering what to do about patients who are at high risk but cannot tolerate statins or refuse to take them.

"Clearly, the focus is to get people on statins," said Dr. Christie Mitchell Ballantyne, the chief of cardiology and cardiovascular research at Baylor College of Medicine, in Houston. "But if someone has seen four doctors and tried six statins and tells me they can’t take them, what am I going to do? Tell them they are a failure?"

Ballantyne said he would give such patients a non-statin drug, despite the guidelines.

Still, some doctors agree that cholesterol targets have been too much of a fixation. Many people, both doctors and patients, have lost sight of the fact that the goal is to reduce the risk of heart disease, not just LDL levels, said Dr. Steven Woloshin, an internist at Dartmouth.

"If you ask patients, ‘Why do you take a statin?’" Woloshin said, "they say ‘to lower my cholesterol level,’ not ‘to lower my cardiovascular risk.’"

Statins do more than just lower cholesterol, noted Dr. Valentin Fuster, director of the heart center at Mount Sinai Hospital in New York City. They also reduce inflammation and blood clotting, both of which are associated with heart attack and stroke risk. Drugs that only reduce LDL have not been shown to be effective in preventing heart attacks.

It is uncertain how much of statins’ effectiveness can be attributed solely to their cholesterol-lowering properties. "Maybe we got lucky with statins," said Dr. Harlan M. Krumholz, a cardiologist at Yale.

The chairman of the committee that developed the new guidelines, Dr. Neil J. Stone of Northwestern University, said the group was prompted to examine the idea of target LDL levels when two doctors — Krumholz and Dr. Rodney A. Hayward of the University of Michigan — asked what the evidence was for their efficacy.

When the committee looked, Stone said, they found no evidence. It was generally accepted that lower was better, but no one had shown that an LDL of 90 milligrams per deciliter, for example, was better than 100. And the high doses and multiple drugs many patients were taking to get to target levels raised concerns.

Dr. Lisa Schwartz, a professor of medicine at Dartmouth, said that medical systems constantly prodded doctors to report patients’ LDL levels and used the numbers to judge physicians’ performance. Referring cardiologists often insist that LDL levels be measured and then lowered.

"Everyone adopted the targets," Schwartz said. "It drove a huge amount of testing and focusing around the LDL number. Many doctors thought it was crazy. We were prescribing higher doses of drugs for older patients, which was probably dangerous."