Diclofenac is a nonsteroidal anti-inflammatory drug (NSAID). This medicine works by reducing substances in the body that cause pain and inflammation. Diclofenac is used to treat mild to moderate pain, or signs and symptoms of osteoarthritis or rheumatoid arthritis. The Cataflam brand of this medicine is also used to treat menstrual cramps. Diclofenac powder (Cambia) is used to treat a migraine headache attack.