The nation’s leading heart experts on Monday issued new guidelines for high blood pressure that mean tens of millions more Americans will meet the criteria for the condition, and will need to change their lifestyles or take medicines to treat it.

Under the guidelines, formulated by the American Heart Association and the American College of Cardiology, the number of men under age 45 with a diagnosis of high blood pressure will triple, and the prevalence among women under age 45 will double.

“Those numbers are scary,” said Dr. Robert M. Carey, professor of medicine at the University of Virginia and co-chair of the committee that wrote the new guidelines.

The number of adults with high blood pressure, or hypertension, will rise to 103 million from 72 million under the previous standard. But the number of people who are new candidates for drug treatment will rise only by an estimated 4.2 million people, he said. To reach the goals others may have to take more drugs or increase the dosages.

Few risk factors are as important to health. High blood pressure is second only to smoking as a preventable cause of heart attacks and strokes, and heart disease remains the leading killer of Americans.

If Americans act on the guidelines and lower their blood pressure by exercising more and eating a healthier diet, or with drug therapy, they could drive an already falling death rate from heart attacks and stroke even lower, experts said.

Now, high blood pressure will be defined as 130/80 millimeters of mercury or greater for anyone with a significant risk of heart attack or stroke. The previous guidelines defined high blood pressure as 140/90. (The first number describes the pressure on blood vessels when the heart contracts, and the second refers to the pressure as the heart relaxes between beats.)

Cardiovascular disease remains the leading cause of death among Americans. The new criteria, the first official diagnostic revision since 2003, result from growing evidence that blood pressure far lower than had been considered normal greatly reduces the chances of heart attack and stroke, as well as the overall risk of death.

Recent research indicates this is true even among older people for whom intensive treatment had been thought too risky. That finding, from a large federal study in 2015, caught many experts by surprise and set the stage for the new revision.

That calculation must be individualized, and experts are recommending that patients use a calculator developed by the guidelines committee at ccccalculator.ccctracker.com.

Nearly half of all American adults, and nearly 80 percent of those aged 65 and older, will find that they qualify and will need to take steps to reduce their blood pressure.

Even under the relatively more lenient standard that had prevailed for years, close to half of patients did not manage to get their blood pressure down to normal.

“Is it going to affect a lot of people, and is it going to be hard to meet those blood pressure goals?” asked Dr. Raymond Townsend, director of the hypertension program at Penn Medicine. “The answer is a pretty significant yes.”

According to the new guidelines, anyone with at least a 10 percent risk of a heart attack or stroke in the next decade should aim for blood pressure below 130/80.

But simply being age 65 or older brings a 10 percent risk of cardiovascular trouble, and so effectively everyone over that age will have to shoot for the new target.

Younger patients with this level of risk include those with conditions like heart disease, kidney disease or diabetes. The new standard will apply to them, as well.

People whose risk of heart attack or stroke is less than 10 percent will be told to aim for blood pressure below 140/90, a more lenient standard, and to take medications if necessary to do so.

If there is any good news for patients here, it is that nearly all the drugs used to treat high blood pressure are generic now. Many cost pennies a day, and most people can take them without side effects.

In formulating the guidelines, the expert committee reviewed more than 1,000 research reports. But the change is due largely to convincing data from a federal study published in 2015.

That study, called Sprint, explored whether markedly lower blood pressure in older people — lower than researchers had ever tried to establish — might be both achievable and beneficial.

The investigators assigned more than 9,300 men and women ages 50 and older who were at high risk of heart disease to one of two targets: a systolic pressure (the higher of the two blood pressure measures) of less than 120, or a systolic pressure of less than 140.

In participants who were assigned to get their systolic pressures below 120, the incidence of heart attacks, heart failure and strokes fell by a third, and the risk of death fell by nearly a quarter.

Those patients ended up taking three drugs on average, instead of two, yet experienced no more side effects or complications than subjects in the other group.

Some experts in geriatrics had expected many more complications among older patients receiving more intense treatment, especially increased dizziness, falls and dehydration.

Instead, intensive treatment reduced the risk of complications related to high blood pressure by more than 30 percent, said Dr. Jeff Williamson, head of the Sticht Center on Aging at Wake Forest Baptist Medical Center and the only geriatrician on the committee drawing up the new guidelines.

With a lower risk of heart attacks and strokes, he noted, the study subjects were more likely to maintain their independence.

But more intensive drug treatment in so many more patients may increase rates of kidney disease, some experts fear. In the Sprint trial, the incidence of acute kidney injury was twice as high in the group receiving drugs to reduce their systolic pressure to 120.

“Although the lower goal was better for the heart, it wasn’t better for the kidney,” said Dr. Townsend of Penn Medicine, who is a kidney specialist. “So yeah, I’m worried.”

While agreeing that lower blood pressure is better, Dr. J. Michael Gaziano, a preventive cardiologist at Brigham and Women’s Hospital and the VA Boston, worries about having doctors and patients fixating on a particular goal.

It’s true, he said, that doctors ought to be more aggressive in treating people at high risk. But, he added, “If a patient comes in with a blood pressure of 180, I will not get him to 130.”

Lifestyle changes like diet and exercise can help many patients lower blood pressure. But many of the newly diagnosed are likely to wind up on drugs, said Dr. Harlan Krumholz, a cardiologist at Yale University.

“This is a big change that will end up labeling many more people with hypertension and recommending drug treatment for many more people,” he said.

The current treatment strategy has not been so successful for many patients, he noted.

“How they tolerate drugs, whether they want to pursue lower levels, are all choices and should not be dictated to them,” said Dr. Krumholz. “Or we will have the same situation as today — many prescriptions that go unfilled and pills untaken.”