The nation’s leading heart organizations released new guidelines on Tuesday that will fundamentally reshape the use of cholesterol-lowering statin medicines, which are now prescribed for a quarter of Americans over 40. Patients on statins will no longer need to lower their cholesterol levels to specific numerical targets monitored by regular blood tests, as has been recommended for decades. Simply taking the right dose of a statin will be sufficient, the guidelines say.

The new approach divides people needing treatment into two broad risk categories. Those at high risk because, for example, they have diabetes or have had a heart attack should take a statin except in rare cases. People with extremely high levels of the harmful cholesterol known as LDL — 190 or higher — should also be prescribed statins. In the past, people in these categories would also have been told to get their LDL down to 70, something no longer required.

Everyone else should be considered for a statin if his or her risk of a heart attack or stroke in the next 10 years is at least 7.5 percent. Doctors are advised to use a new risk calculator that factors in blood pressure, age and total cholesterol levels, among other things.

“Now one in four Americans over 40 will be saying, ‘Should I be taking this anymore?’ ” said Dr. Harlan M. Krumholz, a cardiologist and professor of medicine at Yale who was not on the guidelines committee.