Diclofenac is a non-steroidal anti-inflammatory drug (NSAID) used to relieve pain and inflammation. A substance in the body called cyclo-oxygenase (COX) is involved in the production of various chemicals in the body, including prostaglandins. Prostaglandins are produced in response to injury and certain diseases and conditions, and cause pain, swelling and inflammation. Diclofenac blocks the action of COX, so stopping the production of these prostaglandins. This results in less pain, swelling and inflammation.