Aspirin. Computer graphic of a molecule of aspirin or acetylsalicylic acid, a pain-killing (analges- ic), anti-inflammatory drug used to treat fever, injuries, arthritis and heart disease. The molecule's atoms are colour-coded: carbon (C) is grey, hydrogen (H) is white and oxygen (O) is red. Aspirin has the chemical formula C9H8O4. The drug works by blocking the action of the cyclooxygenase enzymes which produce hormone-like prostaglandin fatty acids. Some of these prostaglandins sensitise nerve endings and dilate blood vessels after an injury. Large doses of aspirin over long periods can lead to stomach and intestinal ulcers.