Aspirin, or acetylsalicylic acid , (acetosal) is a drug in the family of salicylates, often used as an analgesic (to relieve minor aches and pains), antipyretic (to reduce fever), and as an anti-inflammatory. It also has an antiplatelet ("blood-thinning") effect and is used in long-term, low doses to prevent heart attacks and thrombus formation in hypercoaguable states (e.g.in cancer). Low-dose, long-term aspirin use irreversibly blocks the formation of thromboxane A2 in platelets, producing an inhibitory effect on platelet aggregation. This anticoagulant property makes it useful for reducing the incidence of heart attacks. Aspirin produced for this purpose often comes in 75 or 81 mg, dispersible tablets and is sometimes called "junior aspirin" or "baby aspirin." High doses of aspirin are also given immediately after an acute heart attack. These doses may also inhibit the synthesis of prothrombin and may, therefore, produce a second and different anticoagulant effect, but this is not well understood. Its primary, undesirable side-effects, especially in higher doses, are gastrointestinal distress (including ulcers and stomach bleeding) and tinnitus. Another side-effect due to its anticoagulant properties is increased bleeding in menstruating women. Because there appears to be a correlation between aspirin and Reye's syndrome in children under the age of about 12, aspirin is no longer used to control flu-like symptoms or the symptoms of chickenpox in minors.

Aspirin was the first-discovered member of the class of drugs known as non-steroidal, anti-inflammatory drugs (NSAIDs), not all of which are salicylates, though they all have similar effects and a similar action mechanism.