Instead, the research found, many doctors are opting to prescribe the more expensive, heavily marketed statin drugs, which are no more effective than aspirin in preventing heart disease.
"What really concerns us is the degree of aspirin underutilization among patients who have already had a heart attack or a stroke because this is the group for which there is conclusive clinical evidence that aspirin reduces the risk for cardiovascular disease," said Jun Ma, MD, PhD, research associate at the Stanford Prevention Research Center and senior author of the study that appears in the November issue of the Public Library of Science-Medicine.
Of those with the highest risk for cardiovascular disease, the study showed that only 33 percent of their outpatient visits were associated with aspirin therapy in 2003. That's an increase from the 1993 rate of 22 percent, but Ma said the gain is minimal considering the abundant proof of aspirin's benefits for such patients.
And although aspirin use remains low, the study found that U.S. physicians are increasingly prioritizing a class of drugs known as statins as the primary drug strategy for preventing heart disease. Statins, while as effective as aspirin in preventing heart attacks, cost substantially more. The researchers speculate that the heavy marketing of statins in the United States is spurring patients and doctors toward these newer - although not necessarily better - medications.
"Our concern is that physicians are failing to use the full range of strategies available to reduce patients' risk of heart disease and stroke," said Randall Stafford, MD, PhD, associate professor of medicine at the Stanford Prevention Research Center and the first author of the study.
Cardiovascular disease remains the leading cause of death in the United States for both men and women. The American Heart Association estimates that the various types of cardiovascular disease, including heart attack and stroke, affect more than 70 million Americans.
Aspirin therapy is one way for patients diagnosed with a significant risk for heart disease to lower that risk. Aspirin reduces the clotting action of the blood's platelets, thereby lowering the risk of heart attack, stroke and other diseases that can occur when the blood vessels are blocked by clots. In several clinical trials, daily doses of between 75 and 325 milligrams of aspirin cut the risk of cardiovascular events by 15 to 40 percent. However, aspirin can cause gastrointestinal bleeding in up to 4.5 percent of patients; physicians and patients must weigh that risk against aspirin's benefits before deciding whether to use it.
For several years, clinical guidelines have recommended aspirin therapy for patients with a significant risk of heart disease. Ma and Stafford said their study is the first to examine how aspirin use varies among U.S. outpatients according to their heart-disease risk.
The researchers used two national databases that tracked outpatient visits to hospitals and physician offices between 1993 and 2003, and then assessed the reported use of aspirin therapy associated with those visits. Patients were categorized according to their risk of cardiovascular disease. High-risk patients were those who had been diagnosed with heart disease or had experienced a heart attack or stroke. Intermediate-risk patients were divided into two groups: one for those who had diabetes and additional risk factors for heart disease (such as high blood pressure, high cholesterol and smoking); and the other for non-diabetic adults with at least two risk factors for heart disease. All other patients were placed in the low-risk category.
While less than a third of the high-risk patient visits were associated with aspirin therapy, the 2003 rates were even lower for those in the intermediate groups: 12 percent of the diabetic group and 16 percent for the non-diabetics.
Ma and Stafford acknowledged that measuring aspirin usage is difficult because doctors don't always document the over-the-counter medications their patients take. However, based on previous studies that found low rates of aspirin use, the researchers said they don't believe such under-reporting would account for the huge gap between the clinical recommendations and what they found in the new study.
One finding that intrigued the researchers is the growing popularity of statins, which help reduce the amount of LDL or "bad" cholesterol in the blood. Statins and aspirin have similar overall rates of effectiveness in reducing cardiovascular risk, but they vary significantly in cost. A daily dose of the most frequently prescribed statin medication costs around $2 while a daily dose of aspirin costs mere pennies.
The study was funded by an unrestricted grant from Bayer Pharmaceutical Corp., which makes Bayer aspirin, and by the Agency for Healthcare Research and Quality. Also contributing to the study was Veronica Monti, a research dietitian at the Stanford Prevention Research Center.
Stanford University Medical Center integrates research, medical education and patient care at its three institutions - Stanford University School of Medicine, Stanford Hospital & Clinics and Lucile Packard Children's Hospital at Stanford. For more information, please visit the Web site of the medical center's Office of Communication & Public Affairs at http://mednews.