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Platelets are a type of blood cell. Their sticky surface allows them to begin the process that forms blood clots. Blood clots are important to stop bleeding. But they can also block blood flow to the heart and trigger a heart attack. Or they can block arteries to the brain. This may cause an ischemic stroke, the most common type of stroke. Aspirin makes the platelets less sticky. This lowers the risk for blood clots. If you stop taking aspirin, its effect continues for about one week. That's why providers often advise that a person scheduled for surgery or certain procedures stop taking aspirin one week before. This lowers the risk of bleeding after surgery or other invasive procedures.

It's important to talk with your doctor about the possible benefits and drawbacks of taking aspirin before you start to take it. Don't take it on your own without first talking with your provider. For most adults who have never had a past heart attack or stroke, it's no longer advised to take aspirin to prevent a first heart attack or stroke. Aspirin may be considered for certain adults ages 40 to 59 who are at a higher risk for heart attack or stroke, but who are not at increased risk for bleeding. Aspirin may be taken if you have had a past heart attack or stroke. Or if you have had coronary artery bypass surgery. It's also used after a procedure called a stent placement. A tiny wire mesh tube, or stent, is placed in an artery to keep it open. Aspirin helps prevent blood clots from forming on the stent. In some people, taking aspirin does pose health risks. These include people who have peptic ulcers, digestive bleeding, or allergic reactions. 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 blood-thinner (anticoagulant) medicine such as warfarin or clopidogrel, aspirin may further raise the risk of bleeding. So it's important that your doctor knows if you are taking either or both medicines. Don't start aspirin without your provider's input.

Some people may be resistant to aspirin. In these people, aspirin may not give the same protection that it will to those who aren’t resistant. In that case, doctors may prescribe another medicine that helps prevent sticky platelets.

Current research says that aspirin doesn’t affect how ACE inhibitors work. In the past, some health experts thought that aspirin kept ACE inhibitors from working as well as they should. But studies have disproved that. ACE inhibitors lower blood pressure by blocking an enzyme that helps narrow blood vessels. These medicines are often given to people after a heart attack or to people with heart failure.

Doctors in the past old bypass patients to not take aspirin after surgery. But research suggests that taking aspirin within 48 hours of coronary bypass surgery increases the chance of survival. It also reduces the rate of complications of the heart, brain, kidneys, and digestive tract.

Blood vessels that are affected by atherosclerosis and are narrowed by plaque become inflamed and not flexible. Clinical research has found that aspirin helps prevent blood clots from forming. 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. This is the pain or discomfort in the chest caused when the heart muscle does not get enough blood. Aspirin eases 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 strokes are caused by blood vessels that break open (rupture). Taking aspirin could possibly make these bleeding (hemorrhagic) strokes worse. Aspirin may also help to prevent more ischemic strokes, which often happen shortly after the first one. Aspirin given over a long time may also help lower the risk for a second ischemic stroke. But it may raise the risk for a hemorrhagic stroke or bleeding. Aspirin can be combined with other medicines that decrease the stickiness of platelets. An example is ticagrelor. This combo decreases the risk for a second ischemic stroke. The antiplatelet medicines are started within 24 hours of when the stroke symptoms begin.