Up to one in four people aren’t getting the expected heart benefits…

For millions of Americans, popping one or two daily low-dose (81 mg) aspirin pills is an easy way to help prevent heart attacks and strokes.

What most people don’t realize: Even though this type of “aspirin therapy” has been used since the 1970s in the US, the majority of patients—as well as many doctors—aren’t aware that the drug may not be doing the job for all of those taking it. That’s because 15% to 25% of Americans are “aspirin-resistant” and may not fully respond to the drug’s cardioprotective properties.

It’s troubling because aspirin (typically 81 mg to 162 mg daily) is routinely prescribed for heart patients. Research shows that daily aspirin therapy can reduce the risk for second heart attacks in men and women by about 25% and the risk for second strokes by about 22%.

Recent development: Even though the evidence has long been unclear whether aspirin helps prevent a first heart attack or stroke in both sexes, the American Heart Association has recently recommended that low-dose aspirin therapy be considered in adults age 50 and older who have a 10-year cardiovascular risk of at least 10% but no increased risk for bleeding.* (Uncontrolled high blood pressure and diabetes are among the factors that increase one’s 10-year risk.)

Those who are aspirin-resistant, however, face the risks associated with aspirin therapy (mainly gastric bleeding and, in rare cases, hemorrhagic stroke)—without any appreciable gain in long-term health. Key facts about aspirin therapy…


A daily aspirin prevents blood platelets from clumping and forming clots—the cause of most heart attacks and strokes. But what happens when it doesn’t work?

Troubling research: A recent study found that aspirin-resistance in some patients could be linked to a tripling (or more) of major cardiovascular events. This doesn’t mean that aspirin isn’t an effective treatment. It does work for the majority of patients who take it for its cardioprotective benefits.

But if you don’t respond to aspirin…or respond just a little, there  may still be a solution.**


Some experts believe that aspirin resistance may not be due to physiological reasons—but instead mainly from improper use.

Example: Many people who take aspirin for cardiovascular health also take other nonsteroidal anti–inflammatory drugs (NSAIDs)—such as ibuprofen (Motrin) or naproxen (Aleve)—for unrelated conditions. These drugs are believed to antagonize (block) the clot-inhibiting effects of aspirin. Other possibilities…

  • Age. Aspirin resistance is more common in those who are age 75 or older. Age-related declines in stomach acid may limit the absorption of aspirin and make it less effective.
  • Aspirin coatings. A study in the journal Circulation found that the enteric coating on aspirin, used to keep the drug from dissolving in the stomach and ease pain and bleeding for some people with gastritis or ulcers, slowed the effects on platelets. Patients still responded to the treatment, but it took longer (about eight hours) for the drug to be fully absorbed and active.
  • Insufficient dosing. An analysis of aspirin research by the Antithrombotic Trialists’ Collaboration, led by the UK’s University of Oxford, concluded that doses lower than 75 mg (commonly used in the UK) might be ineffective. One study found that about 27% of aspirin-resistant patients were taking lower doses than those without resistance. The most effective dose seems to be 75 mg to 150 mg daily, the UK research found.
  • Other causes. Some patients might have genetic factors that reduce aspirin’s effectiveness. Other patients might have high platelet turnover, leaving a smaller percentage of aspirin-affected platelets in the bloodstream. And some might have metabolic pathways that allow platelets to “activate,” thus promoting blood clots, even in the presence of aspirin.


Blood tests can readily detect the effects of aspirin on platelet aggregation, part of the sequence that leads to clotting. The tests are routinely performed on patients who are scheduled for stents or other cardiac procedures. However, they’re not commonly used to check for aspirin resistance.

My advice: Anyone who takes aspirin to prevent heart attack or stroke should ask his/her doctor if platelet function testing is needed—and how often. The tests are quick and typically covered by insurance. Your doctor needs to know if you’re not responding to aspirin so that he can recommend other approaches (such as those described below) to reduce your clot risks. What helps…

  • Use plain aspirin. As previously explained, research suggests that the rate of aspirin resistance is higher in people who take enteric-coated aspirin. So take plain, uncoated aspirin instead if it doesn’t upset your stomach.
  • Consider a higher dose. In some patients, a dose of 162 mg a day (or even higher) might be more effective, especially if they’ve had a recent heart attack or stroke. Since higher-dose aspirin can increase GI complications, take more only if your doctor recommends it.
  • Divide the dose. There’s some evidence that taking aspirin in two doses—once in the morning and again in the evening—can reduce resistance. You would take the same daily dose that you did before. Just divide it in two—for example, 81 mg in the morning and 81 mg at night.
  • Space your medicines. If you are taking aspirin for heart health, try to avoid ibuprofen and similar drugs. If this isn’t possible, space the doses—take the nonaspirin NSAID at least two hours before (or two hours after) your daily aspirin.
  • Switch drugs. Patients who do not respond to aspirin can get similar cardiovascular protection from clopidogrel (Plavix), ticlopidine (Ticlid) or other blood thinners. Your doctor might recommend aspirin and one of these drugs. Using them together may reduce the resistance to aspirin.
  • Follow directions! About three-quarters of patients do not. In a study of 136 coronary-artery stent patients, 14% were found to be unresponsive to aspirin therapy one month after discharge. However, all but one patient became responsive after they made a commitment to take aspirin daily, as prescribed.

*To calculate your 10-year risk for heart attack or stroke, go to CVRiskCalculator.com.

**Important: Never start or stop aspirin therapy without consulting your doctor.