Indomethacin is a nonsteroidal anti-inflammatory analgesic used in the treatment of disorders such as rheumatoid arthritis, ankylosing spondylitis, and osteoarthritis. Prostaglandins have a significant stimulatory effect on established labor; because indomethacin is a potent inhibitor of prostaglandin synthesis, this agent is also used in the treatment of preterm labor. Like other prostaglandin synthetase inhibitors, indomethacin acts by inhibiting the activity of the cyclo-oxygenase enzyme necessary for the synthesis of prostaglandins, prostacyclin, and thromboxane. Unlike aspirin, which causes an irreversible inhibition of this enzyme, indomethacin results in a competitive and reversible inhibition.