For years, doctors have been using a mathematical equation to assess patients’ risk of experiencing a cardiovascular disease event. Called the Atherosclerotic Cardiovascular Risk Equation, the formula assesses an individual’s risk for events such as a heart attack, ischemic stroke, or death from coronary artery disease. Though the equation has been considered a monumental step toward preventing heart disease, a recent study suggests the equation is flawed and has consequently led many patients to be prescribed unnecessary medications.

Published in the Journal of the American College of Cardiology, the study states that the risk equation substantially overestimates adults’ actual risk for a condition called atherosclerosis. Also known as atherosclerotic cardiovascular disease, atherosclerosis occurs when cholesterol plaque builds on the walls of the arteries. Eventually, the plaque can obstruct blood flow, triggering a heart attack or stroke and killing the person.

The risk equation gained popularity as a method of predicting atherosclerosis after it was published by the American College of Cardiology in 2013. However, it was wildly outdated. First developed in the 1990s, the risk equation was based on a group of volunteers with little ethnic diversity and a small age range, making the equation extremely inaccurate when used across different groups of people.

“Our study provides critical evidence to support recalibration of the risk equation in real world populations, especially given the individual and public health implications of the widespread application of this risk calculator,” said senior author Dr. Alan S. Go, chief of cardiovascular and metabolic conditions research at the Kaiser Permanente Northern California Division of Research, in a press release.

To determine whether the risk equation could be improved by incorporating data from different ethnic and age groups, the researchers studied the cholesterol levels of a large, multi-ethnic population for five years. Over 300,000 men and women between the ages of 40 and 75 were included in the study.

The researchers found the actual incidence of atherosclerotic cardiovascular disease events in these participants was substantially lower than their predicted risk from the Atherosclerotic Cardiovascular Risk Equation. For groups with a predicted risk equal to or greater than five percent, the actual incidence was 1.85 percent. For predicted risk less than 5 percent, 3.75 percent, and 2.5 percent, the actual incidences were 0.9 percent, 0.65 percent, and 0.2 percent, respectively.

“From a relative standpoint, the overestimation is approximately five to six-fold,” Go said. “Translating this, it would mean that we would be overtreating a good many people based on the risk calculator.”

For patients who the equation suggests have the highest risk of developing atherosclerosis, doctors prescribe statins, a class of drugs that maintain the body’s cholesterol levels. Though statins have proved to be quite effective in preventing heart disease, the drugs have a lot of side effects, and it is best not to take them if they are unnecessary.

“Statin therapy is a mainstay treatment for millions of Americans,” said lead author Dr. Jamal S. Rana, a cardiologist at Kaiser Permanente Oakland Medical Center and an adjunct investigator with the Division of Research. “Our study highlights the importance of ongoing research and dialogue in this area to provide more rigorous evidence to guide treatment for the patients most likely to benefit from this approach.”