Aspirin is probably the most common over-the-counter medication, as it blocks the production of cyclooxygenase (COX) enzymes, COX-1 and COX-2. These two important enzymes have different functions: COX-1 catalyzes the biosynthesis of hormones important in maintaining the stomach lining; COX-2 catalyzes the biosynthesis of chemicals that promote inflammation, fever, and pain when an injury occurs. The positive effects of aspirin (pain and inflammation reduction) are due to the blocking of COX-2 enzymes; however, the negative aspects of aspirin (stomach problems) are due to the blocking of COX-1 enzymes.