More than nine million people may miss out on cholesterol-lowering drugs that prevent heart attacks and strokes if doctors choose one set of medical guidelines over another, according to a new study.

That's because the government-backed U.S. Preventive Services Task Force (USPSTF) set a higher threshold for use of the drugs, known as statins, than the American College of Cardiology and the American Heart Association (ACC/AHA).

"I would say we’re still searching for the perfect guidelines," said lead author Michael Pencina, of Duke University in Durham, North Carolina.

The 2013 ACC/AHA guidelines recommend statins for people ages 40 to 75 with at least a 7.5 percent risk of having a heart attack or stroke in the next 10 years. (The ACC/AHA cardiovascular risk estimator tool is available online here: http://bit.ly/2pPwoXh.)

The ACC/AHA also recommends statins for people with cardiovascular disease, for diabetics between ages 40 and 75 and for adults with high levels of “bad” low-density lipoprotein cholesterol.

The 2016 USPSTF recommendation endorses statins for people ages 40 to 75 with at least a 10 percent or greater risk of a heart attack or stroke over the next decade and at least one cardiovascular risk factor like diabetes or high blood pressure.

Pencina told Reuters Health fewer people would be using statins under the more conservative USPSTF guidelines. "What we wanted to do is quantify the impact and look at what it means in terms of numbers."

The researchers applied the recommendations to nationally representative data collected from 3,416 people without a history of cardiovascular disease between 2009 and 2014.

Overall, 21.5 percent were already on statins to prevent heart attacks and strokes.

An additional 24.3 percent would be on statins if all doctors followed the ACC/AHA guidelines, compared to an additional 15.8 percent if all doctors followed the USPSTF recommendation.

The difference between the two guidelines represents about 9.3 million people in the United States, the researchers write in JAMA.

Under the USPSTF guidelines, some diabetics would be excluded from statin use. More than half of those excluded would be middle-aged adults with a more than 30 percent average risk of a cardiovascular event over the next 30 years.

"About one in three people are going to experience a cardiovascular event over the next 30 years," said Pencina.

In a statement to Reuters Health, the USPSTF said its recommendations are based on the best available evidence about a preventive service's benefits and harms.

"Because the USPSTF makes recommendations that are closely tied to the available evidence, we focused on recommending statins for the people who the evidence showed were most likely to benefit, though ultimately this decision should be made through a conversation between each patient and their doctor," the statement continued.

In its review of evidence, the USPSTF focused on 19 trials involving a total of 71,344 people who had no history of cardiovascular disease. Overall, people were 14 percent less likely to die during the study period if they were taking statins than if they were taking a dummy pill or nothing at all.

The risk of serious side effects from statins was also low.

The USPSTF is always more conservative in its recommendations than professional organizations - not just for cholesterol, said Dr. Steve Nissen, chairman of the Robert and Suzanne Tomsich Department of Cardiovascular Medicine at the Cleveland Clinic.

"Whether you treat or not treat is frankly something that should be a discussion between patient and physician," he told Reuters Health. "That’s how I do it."

Nissen, who was not involved in the new study, said some entity should step in to clear up the confusion between the USPSTF, ACC/AHA and several other statin guidelines.

"I’m not terribly happy to have multiple guidelines floating around out there," he said.

Pencina said it's important for patients to be informed about their risk of cardiovascular disease and understand the risks and benefits of statins.

"Both sets of guidelines - to their credit - recommend an informed decision between the patient and the clinician," he said. "Those are crucial."