By GINA KOLATASEPT. 11, 2015

The study found that patients who were assigned to reach a systolic blood pressure goal below 120 — far lower than current guidelines of 140, or 150 for people over 60 — had their risk of heart attacks, heart failure and strokes reduced by a third and their risk of death reduced by nearly a quarter.

The study, called Sprint, randomly assigned more than 9,300 men and women ages 50 and over who were at high risk of heart disease or had kidney disease to of two systolic blood pressure targets: less than 120 millimeters of mercury, which is lower than any guideline ever suggested, or less than 140. (Systolic pressure is the higher of the two blood pressure numbers and represents pressure on blood vessels when the heart contracts.)

The study was expected to conclude in 2017, but considering the results of great importance to public health, the National Heart, Lung and Blood Institute announced them Friday morning, saying a paper with the data would be published within a few months.

“This study provides potentially lifesaving information,” Dr. Gary H. Gibbons, director of the institute, said in a statement announcing the decision.

Nearly 79 million adults in this country — one of three — have high blood pressure, and half of those being treated for it still have systolic pressures over 140.

“This study will shake things up,” predicted Dr. J. F Michael Gaziano, a professor of medicine at Harvard who was not involved with the study. He anticipated that it would have the same effect on people’s thinking about blood pressure as studies of about lowering cholesterol levels did when they showed that, contrary to what many had thought, the lower the number the better.

“It is outstanding news,” said Dr. Mark Creager, president of the American Heart Association and director of the Heart and Vascular Center at Dartmouth-Hitchcock Medical Center, who was not involved in the study. “It will serve as a road map and will save a significant amount of lives.”

If guidelines are changed because of this study — as blood pressure experts expect that they will be — an already falling death rate from heart attacks and stroke could drop even more, said Dr. Jackson T. Wright Jr., a blood pressure expert at Case Western Reserve University and University Hospitals Case Medical Center, and a study investigator. Because cardiovascular disease is still the leading cause of death in the United States, a change in blood pressure goals could also reduce the nation’s overall mortality rate, he said.

The study ventured into unknown territory that some had found a bit frightening. A systolic pressure that is naturally 120 might be good, but it is quite another matter to artificially drag pressure down so low with drugs.

Reaching a target that low would mean giving people more and more medications, and the side effects could cancel any benefit. Older people might be especially vulnerable to ill effects of a much lower blood pressure since many already take an array of drugs for chronic conditions, which might interact. A very low blood pressure could lead to dizziness and falls. Twenty-eight percent of the subjects in the new study were over age 75.