Aspirin decreases the risk of heart attack and stroke by reducing the risk of harmful clot formation. When you cut or injure yourself, the bleeding is stopped by plugging the damaged blood vessels with blood clots. These blood clots are formed by small blood cells known as platelets. Clot formation is triggered by the injured areas of the blood vessels where platelets stick together to form a clot.
Aspirin works by inhibiting platelet aggregation and the formation of thrombi in arteries. Other antiplatelet drugs are clopidogrel, prasugrel and ticagrelor. Aspirin in cardiovascular prevention is used in a low dose of 75mg daily, much less than doses of 500-1000mg used in analgesia (as a pain killer).
Aspirin should not be used in children below the age of 16. Exemptions include certain situations after heart surgery and Kawasaki disease when aspirin can be given on specialist recommendation.
The benefit of aspirin is most significant immediately after a coronary event, revascularisation or stroke, but the treatment is usually continued lifelong. Aspirin used to be recommended in primary prevention, i.e. in patients at high risk of heart attack or stroke (e.g. in diabetes or hypertension) before they had the event. However, the beneficial effect is only minimal and is largely offset by an increased risk of bleeding, therefore aspirin is no longer recommended in this scenario.