The nation’s first new guidelines in a decade for preventing heart attacks and strokes call for twice as many Americans — one-third of all adults — to consider taking cholesterol-lowering statin drugs.

The guidelines, issued Tuesday by the American Heart Association and American College of Cardiology, are a big change. They offer doctors a new formula for estimating a patient’s risk that includes many factors besides a high cholesterol level, the main focus now. The formula includes age, gender, race and factors such as whether someone smokes.

The guidelines for the first time take aim at strokes, not just heart attacks. Partly because of that, they set a lower threshold for using medicines to reduce risk.

The definition of high cholesterol isn’t changing, but the treatment goal is. Instead of aiming for a specific number, using whatever drugs get a patient there, the advice stresses statins such as Lipitor and Zocor and identifies four groups of people they help the most.

“The emphasis is to try to treat more appropriately,” said Dr. Neil Stone, the Northwestern University doctor who headed the cholesterol guideline panel. “We’re going to give statins to those who are the most likely to benefit.”

Doctors say the new approach will limit how many people with low heart risks are put on statins simply because of a cholesterol number. Yet under the new advice, 33 million Americans — 44 percent of men and 22 percent of women — would meet the threshold to consider taking a statin. Under the current guidelines, statins are recommended for only about 15 percent of adults.

“It will be controversial, there’s no question about it. For as long as I remember, we’ve told physicians and patients we should treat their cholesterol to certain goal levels,” said the Cleveland Clinic’s Dr. Steven Nissen. “There is concern that there will be a lot of confusion about what to do.”

The government’s National Heart, Lung and Blood Institute appointed expert panels to write the new guidelines in 2008, but in June said it would leave drafting them to the Heart Association and College of Cardiology.