Aspirin (USAN (United States Adopted Name)), also known as acetylsalicylic acid (pron-en??s?t?l?s�l??s?l?k respell?SET?l-sal-iSILik, abbreviated ASA), is a salicylate drug (medication), often used as an analgesic to relieve minor aches and pains, as an antipyretic to reduce fever, and as an anti-inflammatory medication.

Aspirin also has an antiplatelet (Antiplatelet drug) effect by inhibiting the production of thromboxane, which under normal circumstances binds platelet molecules together to create a patch over damage of the walls within blood vessels. Because the platelet patch can become too large and also block blood flow, locally and downstream, aspirin is also used long-term, at low doses, to help prevent heart attacks (myocardial infarction), strokes, and blood clot (thrombus) formation in people at high risk for developing blood clots. It has also been established that low doses of aspirin may be given immediately after a heart attack to reduce the risk of another heart attack or of the death of cardiac tissue.

The main undesirable side effects (adverse drug reaction) of aspirin are gastrointestinal ulcers (gastric ulcer), stomach bleeding, and tinnitus, especially in higher doses. In children and adolescents, aspirin is no longer used to control flu-like symptoms or the symptoms of chickenpox or other viral illnesses, because of the risk of Reyes syndrome.

Aspirin was the first discovered member of the class of drugs known as nonsteroidal anti-inflammatory drugs (NSAIDs), not all of which are salicylates, although they all have similar effects and most have inhibition of the enzyme cyclooxygenase as their mechanism of action (nonsteroidal anti-inflammatory drugs#Mechanism of action). Today, aspirin is one of the most widely used medications in the world, with an estimated 40,000 tonnes of it being consumed each year.cite journaltitle=Cyclooxygenase-3 (COX-3): filling in the gaps toward a COX continuum?journal=Proc Natl Acad Sci USAdate=2002-10-15first=last=coauthors=Warner TD, Mitchell JA.volume=99issue=21pages=13371-3pmid=12374850 url=T. D.pmc=129677 In countries where Aspirin is a registered trademark owned by Bayer, the generic term is acetylsalicylic acid (ASA).