Aspirin, also known as acetylsalicylic acid (ASA), is a medication used to reduce pain, fever, or inflammation.[5] Specific inflammatory conditions which aspirin is used to treat include Kawasaki disease, pericarditis, and rheumatic fever.Aspirin given shortly after a heart attack decreases the risk of death. Aspirin is also used long-term to help prevent further heart attacks, ischaemic strokes, and blood clots in people at high risk. It may also decrease the risk of certain types of cancer, particularly colorectal cancer. For pain or fever, effects typically begin within 30 minutes. Aspirin is a nonsteroidal anti-inflammatory drug (NSAID) and works similarly to other NSAIDs but also suppresses the normal functioning of platelets.

One common adverse effect is an upset stomach.More significant side effects include stomach ulcers, stomach bleeding, and worsening asthma. Bleeding risk is greater among those who are older, drink alcohol, take other NSAIDs, or are on other blood thinners.Aspirin is not recommended in the last part of pregnancy. It is not generally recommended in children with infections because of the risk of Reye syndrome.High doses may result in ringing in the ears.

A precursor to aspirin found in leaves from the willow tree has been used for its health effects for at least 2,400 years In 1853, chemist Charles Frédéric Gerhardt treated the medicine sodium salicylate with acetyl chloride to produce acetylsalicylic acid for the first time. For the next fifty years, other chemists established the chemical structure and came up with more efficient production methods.[9]:69–75 In 1897, scientists at the Bayer company began studying acetylsalicylic acid as a less-irritating replacement medication for common salicylate medicines.:69–75 By 1899, Bayer had named it "Aspirin" and sold it around the world. Aspirin's popularity grew over the first half of the twentieth century leading to competition between many brands and formulations.The word Aspirin was Bayer's brand name; however, their rights to the trademark were lost or sold in many countries.