Aspirin originated as a herbal remedy, a bitter powder now called salicin which was extracted from the bark of the willow tree in ancient Greek times. This raw form often induced extreme stomach pains as a side effect.  First synthesized as acetylsalicylic acid in 1899, this drug is widely used to treat pain, fever, and inflammation.
Aspirin acts by irreversible inhibition of the enzyme cyclooxygenase. This limits the production of the thromboxanes and prostaglandins. The reduction in thromboxanes, which are responsible for platelet adhesion gives the drug its anticlotting properties. The reduction in prostaglandins alleviates pain and is also responsible for the side effect of ulceration of the gastric mucosa as prostaglandins are stimulators of mucous production in and blood flow to the gastric mucosa.
Because of the anticlotting effect, aspirin is prescribed in low doses—typically 81 mg daily—to help prevent heart attacks, strokes, and blood clot formation in people at high risk for developing blood clots. Research has also shown that aspirin given immediately after a heart attack can significantly increase one's chance of survival.
Aspirin has a number of potential side effects including: ulceration of the stomach lining, easy bruising and bleeding, nausea, and allergic reaction in a relatively small percentage of users. Overdose of aspirin can result in death. Aspirin use in children with some viral illnesses is linked, in some cases, to the development of Reye's syndrome.