For more than 100 years, aspirin has been among the most popular medications for treating mild to moderate pain. Introduced into the United States in 1899, it also reduces inflammation, fever, redness, swelling and discomfort caused by medical disorders such as headaches, infections and arthritis. It also helps reduce clotting of blood. Aspirin belongs to a class of medications known as nonsteroidal anti–inflammatory drugs (NSAIDs).

Aspirin is derived from salicylate, a naturally occurring substance found in the bark of willow trees that has been used for hundreds of years to relieve pain. Scientists are unsure exactly how the substance works, but do know that it prevents pain by acting on the hypothalamus (part of the brain that regulates body temperature and other autonomic activities) and blocking the generation of pain impulses. It also reduces inflammation by inhibiting prostaglandin production. Prostaglandins are hormone–like substances in the body that mediate a wide range of physiological functions.

Aspirin comes in many formulations and dosage levels. Although the drug is largely safe and effective, it can produce dangerous side effects for people with certain health conditions or for special populations, especially children. People are advised to talk with a physician before using aspirin.