aspirin, acetyl derivative of salicylic acid (see salicylate) that is used to lower fever, relieve pain, reduce inflammation, and thin the blood. Common conditions treated with aspirin include headache, muscle and joint pain, and the inflammation caused by rheumatic fever and arthritis. Aspirin is believed to act against fever, pain, and inflammation by interfering with the synthesis of specific prostaglandins in the body. Because of its ability to inhibit the formation of blood clots, aspirin is also used in low doses to prevent heart attack and stroke and to control unstable angina. The drug's usefulness in preventing certain cancers, the dangerous high blood pressure that sometimes occurs during pregnancy (toxemia), and migraine headaches is also under investigation.

Normal dosage may cause nausea, vomiting, diarrhea, or gastrointestinal bleeding. Large doses cause acid-base imbalance and respiratory disturbances and can be fatal, especially in children. Aspirin also has been linked to the development of Reye's syndrome (a combination of acute encephalopathy and fatty infiltration of internal organs) in children who have taken it for viral infections. Acetaminophen (Tylenol), which does not cause gastric irritation but does lower fever and relieve pain, is often substituted for aspirin.

Aspirin, although usually made synthetically now, was originally derived from salicin, the active ingredient in willow bark. Willow bark had been used for centuries in folk medicine in certain parts of the world. Acetylsalicylic acid was first prepared by the German chemist Felix Hoffmann, an employee of Friedrich Bayer & Co., in 1897. It is now the active ingredient in more than 50 over-the-counter preparations; estimates put American consumption at 80 billion tablets annually.

See analgesic.