Aggressive treatment of high blood pressure can sharply cut the risk of heart attacks, strokes and deaths in people 50 and older, according to a landmark federal study released Friday that urges doctors to bring their patients’ blood pressure well below the commonly recommended target.

The new research advises people with high blood pressure to keep their “systolic” pressure — the top number in the reading that health-care providers routinely tell patients — at 120 or below. Clinical guidelines have commonly called for systolic blood pressure of 140 for healthy adults and 130 for adults with kidney disease or diabetes.

Physicians have complained that there was no clear evidence for any specific standard on systolic blood pressure, compelling them to decide on their own how aggressive to be in treating the condition. Now there is a number, and it is significantly lower than the current targets.

“What we didn’t have until today was the proof that being more aggressive was rewarded” with lower incidence of illness and death, said Randall M. Zusman, director of the division of hypertension at the Corrigan Minehan Heart Center at Massachusetts General Hospital. He was not involved in the study.

The research, primarily sponsored by the National Heart, Lung and Blood Institute, showed that using medication to keep systolic blood pressure at 120 or below reduced the rate of heart attacks, strokes and heart failure by a third and the risk of death by almost 25 percent, compared with keeping it at 140. Reaching that conclusion prompted researchers to end the study early and report the results.

Systolic blood pressure is a measure of the pressure in the arteries when the heart muscle contracts. Diastolic blood pressure, the bottom number, is a measure of the pressure in the arteries between heartbeats, when the heart muscle is resting and refilling with blood.

About 70 million U.S. adults — nearly 1 in 3 — have high blood pressure, according to the Centers for Disease Control and Prevention. High blood pressure greatly increases the risk of heart disease and stroke, the leading and third-leading causes of death for Americans, according to the CDC. About half the people being treated for high blood pressure do not have it under control. Genetics, diet and lifestyle influence blood pressure levels.

“This study provides potentially lifesaving information that will be useful to health care providers as they consider the best treatment options for some of their patients, particularly those over the age of 50,” Gary H. Gibbons, director of the institute, which is part of the National Institutes of Health, said in a statement.

That doesn’t mean that doctors will suddenly begin prescribing blood pressure medication to everyone older than 50 with systolic pressure above 120, according to experts.

For one thing, the Systolic Blood Pressure Intervention Trial (SPRINT) examined patients who had a higher-than-normal risk of heart or kidney disease. It enrolled adults over 50 with systolic blood pressures of 130 to 180 and some history of cardiovascular disease, said Natalie Bello, an assistant professor of medicine at Columbia University Medical Center, one of the 100 sites where patients were recruited.

The patients were divided into two groups. One received an average of two medications to keep systolic blood pressure at 140, and the other was given “intensive treatment” to bring blood pressure down to 120 through an average of three medications.

Even if a lower blood pressure is recommended for a patient, physicians will make individual decisions for each person that include consideration of age and health and the possible impact of lifestyle changes and medications, experts said.

“This is going to be a very big step for physicians and patients, because no patient likes taking more medicine, and it’s hard enough to get patients to stay on the medicines they’re on now,” said Howard S. Weintraub, clinical director of the Center for the Prevention of Cardiovascular Disease at New York University.

“If someone is grazing at McDonald’s and eating too much salt and drinking too much alcohol, I can make a difference” by first attempting to alter that lifestyle, said Weintraub, who was not involved in the study. Diet, exercise and relaxation exercises have been shown to lower blood pressure.

Medications that lower blood pressure include angiotensin receptor blockers, angiotensin-converting enzyme inhibitors, diuretics, calcium channel blockers and beta blockers. Almost all of the medications are available as low-cost generics.

With the exception of angiotensin receptor blockers, the medications carry significant side effects, Zusman said.

Diuretics, for example, can lower potassium, resulting in muscle cramps and irregular heartbeats. They can increase blood sugar and cause gout and erectile dysfunction, Zusman said. Beta blockers can cause fatigue, depression, sexual dysfunction and diminished exertional capacity, he said. Sharply lowering an elderly person’s blood pressure can lead to fainting and potentially dangerous falls.

“My challenge is to fashion a drug regimen, along with lifestyle modifications, that successfully lowers their blood pressure to target without interfering with their intellectual or physical capacities,” Zusman said.

In recent years, experts have debated the best blood-pressure targets for older people. As recently as 2013, a panel of the same NIH institute advised that people aged 60 and older should aim for a systolic blood pressure of 150. The American Heart Association considers 120 over 80 or below a normal blood pressure and calls a reading of 120 to 139 in systolic pressure “prehypertension.” Some experts said this is likely to change, but not immediately.

“I would suspect there may be a lag time between when this becomes the law of the land and when people actually embrace it,” Zusman said.

Because the study has not been published, only further analysis will reveal how to treat subgroups such as the elderly and minority groups such as African Americans, who are prone to high blood pressure, Bello said.

More research may have to be done to determine whether the same aggressive approach should be used with people under 50 who develop high blood pressure, including teen-agers, Zusman said.