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 a week. That's why doctors usually recommend that a person scheduled for surgery stop taking aspirin a week before the operation. This lowers the risk of bleeding after surgery.

The American Heart Association recommends that you discuss with your provider the potential benefits and drawbacks of taking aspirin before you start to take it. In some people, taking aspirin does pose health risks. These include peptic ulcers, digestive bleeding, and 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 provider knows if you are taking either or both medicines. Don't start aspirin without you healthcare 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, health care providers 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 medical experts had thought that aspirin keep 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 congestive heart failure.

Healthcare providers in the past had told 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 inflexible and inflamed. 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. Angina 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 ruptured blood vessels. Taking aspirin could potentially make these bleeding (hemorrhagic) strokes worse. According to the American Heart Association, aspirin given within 2 days to a person who has just had an ischemic stroke makes the stroke less severe. Aspirin may also help to prevent more ischemic strokes, which often happen shortly after the first one. Aspirin given over a long period also helps lower the risk for a second ischemic stroke. But it may raise the risk for a hemorrhagic stroke. 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 began.