Acetylsalicylic acid, commonly known as aspirin, is one of the most widely used medications globally, initially recognized for its pain and fever-reducing properties. Its role in preventing stroke is a matter of complex medical evaluation. The decision to use aspirin as a preventive measure is highly dependent on an individual’s specific risk factors and requires a careful balance between benefit and potential harm.
The Anti-Clotting Action of Aspirin
Aspirin’s effectiveness in preventing stroke is tied to its function as an anti-platelet agent, working to prevent blood clots from forming. The majority of strokes are ischemic, caused by a blood clot blocking an artery leading to the brain. Low-dose aspirin, typically 75 to 100 milligrams daily, is sufficient to suppress the clotting mechanism and primarily targets ischemic strokes. Aspirin has no preventative effect on hemorrhagic strokes, which are caused by bleeding in the brain.
The drug achieves this by irreversibly inhibiting a specific enzyme, cyclooxygenase-1 (COX-1), which is present in platelets. This inhibition prevents the production of thromboxane A2 (TXA2), a potent chemical signal that causes platelets to aggregate to form a clot. Because aspirin irreversibly modifies the COX-1 enzyme, the anti-clotting effect lasts for the entire lifespan of the platelet, which is approximately seven to ten days.
Prevention Strategies: Primary Versus Secondary Use
The decision to use aspirin for stroke prevention is divided into two distinct strategies: primary and secondary prevention. Primary prevention refers to taking aspirin to prevent a first-ever heart attack or stroke in people who have not yet experienced a cardiovascular event. Secondary prevention involves using aspirin to prevent a second event in patients who have already had an ischemic stroke, transient ischemic attack (TIA), or heart attack.
For secondary prevention, aspirin is considered a standard course of care for most patients with a history of an ischemic event. In these high-risk individuals, the benefit of reducing the chance of a recurring stroke generally outweighs the risk of bleeding. Continuing low-dose aspirin therapy after a first ischemic stroke is a well-established practice to reduce the risk of another event.
However, current medical guidelines have severely limited the recommendation for aspirin in primary prevention. For adults aged 60 years or older who have no history of cardiovascular disease, the U.S. Preventive Services Task Force (USPSTF) now recommends against initiating low-dose aspirin therapy. This is because the risk of internal bleeding in this older age group often exceeds the benefit of preventing a first stroke.
The decision for adults aged 40 to 59 years with an increased risk of cardiovascular disease is an individualized one, requiring a discussion with a healthcare provider. These individuals must have a specific risk profile, such as a 10% or greater risk of cardiovascular disease over the next ten years, to potentially see a small net benefit from starting low-dose aspirin.
Understanding the Risks of Bleeding
The primary concern with aspirin use stems directly from its anti-clotting action, which increases the risk of bleeding. The two major bleeding risks are gastrointestinal (GI) bleeding and intracranial hemorrhage. Gastrointestinal bleeding can range from minor stomach irritation to severe, life-threatening hemorrhages and ulcers in the stomach or intestines.
Aspirin increases the risk of GI bleeding by two mechanisms: a local effect that irritates the stomach lining and a systemic effect that impairs the blood’s ability to clot and stop any bleeding that occurs. Even low-dose aspirin is associated with an increased risk of GI complications. Certain factors, such as advanced age, a history of ulcers, or the concurrent use of other non-steroidal anti-inflammatory drugs (NSAIDs), can further increase this risk.
Intracranial hemorrhage, or bleeding in the brain, is a more severe, albeit less common, complication of aspirin therapy. This is a type of hemorrhagic stroke, which aspirin is unable to prevent and may actually increase the risk of. This specific risk is a major reason why the use of aspirin for primary prevention is not recommended for the general population.