By- Dr. Erik SteeleAbout one-third of the adult U.S. population — more than 50 million people — take aspirin to prevent heart disease.But doctors are quick to point out that the century-old drug is a double-edged sword. Although aspirin can fight blood clots that can cause heart attacks and strokes, high doses can increase the risk of bleeding. This can lead to gastrointestinal bleeding and strokes caused by bleeding, known as hemorrhagic strokes.But a daily aspirin regimen isn’t healthy for everyone, and doctors say people should take more care when they decide to self-prescribe. They say it’s important to take aspirin daily only in consultation with a doctor or other health care provider.Self-prescribing a daily aspirin regimen is unwise and widespread.General guidelines call for men ages 45 to 79 and women ages 55 to 79 to take aspirin if benefits, such as preventing heart attacks or strokes, outweigh possible problems, such as gastrointestinal bleeding,. Health.com: Should I take aspirin against heart attacks?And while taking an aspirin in the event of chest pain is widely accepted to limit a heart attack, taking an extra aspirin with the onset of stroke-like symptoms can worsen a stroke if it causes bleeding into the area of the brain experiencing the acute stroke.Another reason against self-prescription of aspirin is that for many people, the drug has very little effect.It may be that up to 20 percent of people who take aspirin don’t benefit from it at all, according to research by Dr. Francis Gengo of the Dent Neurologic Institute, and the University at Buffalo, State University of New York.Being resistant to aspirin makes patients four times more likely to suffer a heart attack or stroke or even die from a pre-existing heart condition, compared with nonresistant patients. Health.com: Heart attack risk calculatorSome self-prescribers may be unaware of how aspirin interacts with over-the-counter supplements and herbal remedies. Saint John’s wort has some anti-clotting effects and it may exacerbate the risk of dangerous bleeding if taken along with aspirin.Other patients may forget to take their aspirin, and in some patients, aspirin isn’t absorbed into the bloodstream well. Doctors can use a test to see how well aspirin is being absorbed.”The mechanisms for aspirin resistance are varied, and they’re not really very well understood,” Gengo said. Possible factors include genetic differences, effects of other diseases and blood flow around abnormally narrow vessels. “It can be a whole array of things,” he said.Doctors say that more study is needed on the effects of aspirin and other anti-platelet drugs such as the highly prescribed Plavix. As 20 percent of the U.S. population is expected to be above 65 years old by 2030, finding new ways to combat the risk of heart attacks and strokes could have widespread benefits.