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.