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Platelets are irregularly shaped, colorless cells in blood. Their sticky surface allows them to begin the process that forms blood clots. Blood clots are important for staving off bleeding but can also block the flow to the heart and trigger a heart attack. Or they can block arteries to the brain, possibly leading to an ischemic stroke, the most common type of stroke. Aspirin decreases the stickiness of the platelets, reducing the risk of blood clots. If you stop taking aspirin, its effect continues for about a week. That's why doctors usually recommend that a person scheduled for surgery stop taking aspirin a week before the operation to reduce the risk of bleeding after surgery.

The U.S. Preventive Services Task Force recommends that you discuss with your doctor the potential benefits and drawbacks of taking aspirin BEFORE you start to take it. In some people, taking aspirin does pose health risks, including peptic ulcers, gastrointestinal bleeding, and allergic reactions, and it can increase the risk for a hemorrhagic stroke. Aspirin also may trigger asthma in some people, especially those who have chronic sinusitis and those with nasal polyps. If you take anticoagulant medication such as coumadin, aspirin may further increase the risk of bleeding, so it's important that your doctor knows if you are taking either or both medications.

Some people may be resistant to aspirin. In these people, aspirin may not provide the protection that it will to those who are not resistant. Aspirin blocks the formation of thromboxane A2, a chemical that makes platelets sticky and promotes blood clotting. In someone who is aspirin-resistant, the aspirin doesn't block as effectively the production of thromboxane A2. In that case, doctors may prescribe another medicine that helps prevent sticky platelets.

Current research has shown that aspirin does not affect how ACE inhibitors work. In the past, some medical experts had thought that aspirin would make ACE inhibitors less effective, by restricting the production of hormones that dilate blood vessels. Clinical research studies have disproved that. ACE inhibitors lower blood pressure by blocking an enzyme that helps constrict blood vessels. These drugs are often given to patients after a heart attack or to those with congestive heart failure.

Although doctors traditionally had told bypass patients to avoid aspirin after surgery, clinical research suggests that aspirin within 48 hours of coronary bypass surgery increases the chance of survival and reduces the rate of complications involving the heart, brain, kidneys, and gastrointestinal tract.

Blood vessels that are affected by atherosclerosis and are narrowed by plaque become inflexible and inflamed. Clinical research has found that aspirin not only helps prevent blood clots from forming, but it also protects the blood vessels against even mild inflammation. Inflammation causes changes in blood vessels similar to those seen in people at high risk for heart disease. Inflammation of the blood vessels also is responsible for angina, the pain or discomfort in the chest caused when the heart muscle does not get enough blood. Aspirin relieves angina by reducing the amount of inflammation-producing chemicals in the body.

Most strokes are caused by clots; these are called ischemic strokes. But some are caused by ruptured blood vessels. Taking aspirin could potentially make these bleeding, or hemorrhagic, strokes, worse. According to the American Heart Association, aspirin given within two days to a patient who has just had an ischemic stroke reduces the stroke's severity. Aspirin may also help to prevent subsequent ischemic strokes, which often occur shortly after the first one. Aspirin given over the long term also helps reduce the risk for a second ischemic stroke, but may increase the risk for a hemorrhagic stroke.

A recent report in Circulation, the journal of the AHA, found that only about a quarter of the people who have heart disease and could benefit from aspirin therapy took aspirin to help prevent a recurrent heart attack or other cardiovascular problems. More men than women take aspirin as a preventive measure, possibly because women aren't as aware of the dangers of heart disease as men. A greater percentage of people younger than 80 took aspirin as a preventive measure than the percent of those older than 80. Experts say, however, that aspirin is particularly beneficial in older adults. If you have heart disease, talk with your doctor or health care provider about taking aspirin.